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J Biol Chem. Author manuscript; available in PMC Jan 25, 2006.
Published in final edited form as:
PMCID: PMC1351152
EMSID: UKMS7227

Annexin 2 Binding to Phosphatidylinositol 4,5-Bisphosphate on Endocytic Vesicles Is Regulated by the Stress Response Pathway*

Abstract

Annexin 2 is a Ca2+-binding protein that has an essential role in actin-dependent macropinosome motility. We show here that macropinosome rocketing can be induced by hyperosmotic shock, either alone or synergistically when combined with phorbol ester or pervanadate. Rocketing was blocked by inhibitors of phosphatidylinositol-3-kinase(s), p38 mitogen-activated protein (MAP) kinase, and calcium, suggesting the involvement of phosphoinositide signaling. Since various phosphoinositides are enriched on inwardly mobile vesicles, we examined whether or not annexin 2 binds to any of this class of phospholipid. In liposome sedimentation assays, we show that recombinant annexin 2 binds to phosphatidylinositol 4,5-bisphosphate (PtdIns-4,5P2) but not to other poly- and mono-phosphoinositides. The affinity of annexin 2 for PtdIns-4,5P2 (KD ~5 μm) is comparable with those reported for a variety of PtdIns-4,5P2-binding proteins and is enhanced in the presence of Ca2+. Although annexin 1 also bound to PtdIns-4,5P2, annexin 5 did not, indicating that this is not a generic annexin property. To test whether annexin 2 binds to PtdIns-4,5P2 in vivo, we microinjected rat basophilic leukemia cells stably expressing annexin 2-green fluorescent protein (GFP) with fluorescently tagged antibodies to PtdIns-4,5P2. Annexin 2-GFP and anti-PtdIns-4,5P2 IgG co-localize at sites of pinosome formation, and annexin 2-GFP relocalizes to intracellular membranes in Ptk cells microinjected with Arf6Q67L, which has been shown to stimulate PtdIns-4,5P2 synthesis on pinosomes through activation of phosphatidylinositol 5 kinase. These results establish a novel phospholipid-binding specificity for annexin 2 consistent with a role in mediating the interaction between the macropinosome surface and the polymerized actin tail.

The vertebrate annexin family comprises a group of 12 unique genes that encode proteins which are biochemically characterized by their ability to bind to negatively charged phospholipids in the presence of calcium ions (for a review, see Ref. 1). Although differences exist for individual annexins with regard to preference for specific lipids, such as phosphatidylserine and phosphatidylethanolamine, and also the Ca2+ requirement for half-maximal binding, this fundamental property has led to a paradigm for annexin function in which soluble cytosolic annexins translocate to intracellular membrane surfaces upon elevation of intracellular [Ca2+]. Such Ca2+-dependent membrane association has been demonstrated for several annexins in various cell types (24), although the functional consequences of reversible membrane binding by annexins are not well understood.

Annexin 2 is typical in this regard and has been shown to associate with early endosomes, macropinosomes, and phagosomes (57). However, the interaction between annexin 2 and endosomes is unusual in being Ca2+-independent, instead relying at least in part on the presence of cholesterol in the endosomal membrane (8, 9), although the possible involvement of other protein-lipid and protein-protein interactions cannot be excluded. Annexin 2 is also an F-actin-binding protein, and since endosomes move from their sites of formation to the cell interior in an actin-dependent manner (10, 11), one possible role for annexin 2 in this process is at the interface between the endocytic vesicle and the associated actin filaments. Such a role would fit with the observations that although annexin 2 is a constituent of the actin tails that propel macropinosomes in rat basophilic leukemia (RBL)1 cells, it is enriched at the point of contact between the actin tail and the vesicle (6). Disruption of annexin 2 function by expression of a dominant negative mutant has been shown to disturb the trafficking of endosomes in HeLa cells (12) and to completely inhibit actin-based pinosome motility in RBL cells (6). In the latter study, it was noted that the same annexin 2 mutant had no effect on the actin-dependent motility of the intracellular pathogen, Listeria monocytogenes, demonstrating that the role of annexin 2 lies upstream of the actin polymerization machinery.

These findings indicate that the binding of annexin 2 to endocytic vesicles is key to the function of this protein. Analysis of the lipid composition of endosome, pinosome, and phagosome membranes has revealed that these are enriched in mono- and polyphosphoinositides that are involved in the recruitment of proteins that mediate dynamic changes in the actin cytoskeleton (1315). We show here that actin-dependent macropinosome rocketing is regulated by the stress response pathway, activation of which is known to lead to the synthesis of various phosphoinositides. We therefore examined whether or not annexin 2 could bind to any of this class of lipids, since such interactions may play an important role in the regulation of annexin 2 function. We show that annexin 2 binds with high selectivity to phosphatidylinositol 4,5-bisphosphate (PtdIns-4,5P2) in both the presence and the absence of calcium but not to other polyphosphoinositides tested. We also provide evidence, using a number of different experimental approaches, that annexin 2 binds to PtdIns-4,5P2 in living cells. These findings, when placed in the context of existing knowledge of phospholipid-binding by annexins, demonstrate the versatility of lipid-binding by annexin 2 and support the idea that annexin 2 is a regulator of membrane-cytoskeleton dynamics in vesicle trafficking.

EXPERIMENTAL PROCEDURES

Labeling Macropinosomes with Fluorescently Labeled IgE

Alexa-568 (Molecular Probes) was conjugated to anti-2,4-dinitrophenol IgE (Sigma) according to the manufacturers’ instructions and stored at −80 °C. Adherent RBL cells cultured as described previously (10) were chilled to 4 °C in HEPES-buffered saline (HBS). HBS (150 mm NaCl, 5 mm KCl, 1.8 mm CaCl2, 0.8 mM MgCl2, 20 mM HEPES, 1 mM glucose, pH 7.4) + 100 ng/ml IgE-A568 was added to the cells. After 30 min, cells were washed in ice-cold HBS to remove unbound IgE-A568 and then stimulated to produce actin rockets with hyperosmolar HBS (HBS + 150 mm sucrose), phorbol myristic acid (PMA) (10 nm in HBS), pervanadate (Perv, 200 μm in HBS), or combinations of these agonists for 40 min in 6-well plates at 37 °C. Buffer was replaced with preheated 3.7% formaldehyde in phosphate-buffered saline (PBS). The plate was immediately transferred to room temperature for a further 10 min to complete fixation. Coverslips were then washed three times in PBS, permeabilized for 5 min using PBS supplemented with 10 –100 μg/ml saponin, and washed a further three times in PBS. Cells were incubated overnight at 4 °C in the presence of primary probe and then washed in PBS before incubation with secondary probes for 1 h at 37 °C in a moist chamber. Coverslips were then washed again in PBS before incubation with tertiary probes for 1 h at 37 °C in a moist chamber. Coverslips were washed a further three times and mounted in 90% glycerol, 10% PBS, 0.01% n-propylgallate. To visualize actin rockets, cells were fixed and stained using FITC-phalloidin (Sigma) prior to mounting. To quantify the incidence of rocketing, 100 cells were examined, and the number of cells with one or more clearly defined F-actin rocket was scored. In experiments to examine the pharmacology of rocketing, potential inhibitors and activators were added to cells 20 min prior to stimulation and then maintained at the same concentration throughout stimulation.

DNA Constructs

The annexin 2-GFP and GFP-actin fusion constructs have been described previously (6, 10), as have constructs expressing the constitutively active (Q67L) and dominant negative (T27N) mutants of Arf6 (16).

Liposome Centrifugation Assays

Dioleoylphosphatidylcholine (DOPC,Sigma) alone or mixed with PtdIns3P, PtdIns4P, PtdIns-3,4P2, or PtdIns-4,5P2 (Avanti Polar Lipids) in a molar ratio of 39:1 was made up in 95% chloroform/5% methanol and dried under nitrogen. Lipid films were rehydrated by vortexing in 250 mm raffinose, 25 mm HEPES, 1 mm dithiothreitol, pH 7.4, and extruded 15 times through a polycarbonate membrane, pore size 0.2 μm (Whatman), to produce optically clear suspensions of small, unilamellar liposomes (17). These were diluted with three volumes of binding buffer (BB) (100 mm sorbitol, 40 mm HEPES, 1 mm dithiothreitol, 130 mm KCl, 0.5 mm MgCl2, pH 7.1) and pelleted at 35,000 rpm for 10 min (S120AT2, Sorvall). Liposomes were then gently resuspended in BB supplemented with 1 mm EGTA, or various concentrations of Ca2+, to give a final total lipid concentration of 3.96 mm. 25-μl dilutions of liposomes in BB were gently mixed with 5 μg of annexin 2 purified to > 95% homogeneity and diluted into 25 μl of BB to give a final concentration of annexin 2 of 2.78 μm. In most experiments, liposomes were diluted such that the phosphatidylinositide was present at 18-fold molar excess with respect to the annexin. Mixtures of annexin 2 and liposomes were incubated for 1 h at 20 °C and centrifuged at 50,000 rpm for 10 min (S100AT3, Sorvall). The uppermost 40 μl of supernatant was removed, and the pellet was gently washed with 100 μl of BB and centrifuged at 50,000 rpm for a further 10 min. The top 90 μl was again gently removed, and the pellet was resuspended in 25 μl of SDS-PAGE loading buffer. The samples were separated by SDS-PAGE, and the annexin was visualized by staining with Coomassie blue. Gels were destained, and the bands were quantitated by densitometry using a Fujifilm LAS-1000 imager and AIDA-230 software.

Loading of Cells with PtdIns-4,5P2-TMR, Rocket Induction, and Sub-sequent Fixation

PtdIns-4,5P2-TMR (Molecular Probes) was loaded into RBL cells using a Shuttle PIPTM carrier-1 (Molecular Probes) according to the manufacturer’s instructions. Briefly, 40% confluent RBL cells grown on glass coverlips (Matek) were incubated with 50 μl of growth media (Dulbecco’s modified Eagle’s medium + 10% fetal calf serum) containing 3 μl of Shuttle-PIP/TMR-PtdIns-4,5P2 complexes. Complexes were allowed to form by incubating 2 μl of 1 μg/μl TMR-PtdIns-4,5P2 with 3 μl of 0.5 mm Shuttle PIP stock for 5 min at 20 °C. To induce rocketing vesicles, cells were exposed to 150 mm sucrose, 10 nm PMA in Hanks’-buffered saline solution for 15 min as described previously (10). Cells were washed twice with prewarmed Hanks’-buffered saline solution and fixed for 30 min with prewarmed 4% paraformaldehyde in Dulbecco’s modified Eagle’s medium, 2 mm EGTA, and for a subsequent 3 h in 4% paraformaldehyde in PBS at 4 °C.

Indirect Immunofluorescence

RBL cells, loaded with PtdIns-4,5P2-TMR and fixed as described above, were incubated with the monoclonal mouse anti-annexin 2 antibody HH7 in 0.2% saponin, 1% fetal calf serum, 50 mm glycine, 0.1% BSA in PBS overnight at 4 °C. Cells were washed four times with PBS, and the secondary antibody was added in PBS + 0.2% saponin. Actin was stained using AlexaFluor 545-phalloidin (Molecular Probes) in PBS + 0.2% saponin. Images were obtained using a Radiance 2000 confocal microscope (Bio-Rad) and processed using MetaMorph.

PIP Strips

PIP Strips were obtained from Echelon Biosciences Inc. These are nitrocellulose membranes onto which phospholipids have been immobilized (100 pmol/spot). Strips were blocked with 2% bovine serum albumin (fraction V) in TTBS (50 mm Tris, pH 8.0, 150 mm NaCl, 0.1% Tween 20, ± EGTA/Ca2+) and incubated with annexin 2 in TTBS overnight at 4 °C. Blots were washed four times with TTBS, and bound annexin 2 was visualized with conventional indirect immunoblotting using the monoclonal antibody HH7 and ECL (Amersham Biosciences).

Purification of Recombinant Proteins

Annexins 1 and 2 were purified from porcine intestine and yeast, respectively, as described previously (18, 19). Annexin 5 was affinity-extracted from bacterial cell lysates as a fusion with glutathione S-transferase and purified following thrombin cleavage by ion exchange chromatography.

Microinjections

The anti-PtdIns-4,5P2 antibody (a kind gift of Dr. G. Schiavo, Cancer Research, UK) was labeled using an AlexaFluor 555 protein labeling kit (Molecular Probes, Eugene, OR) and microinjected (1 mg/ml in phosphate buffer, pH 7.1) into RBL cells at room temperature with an Eppendorf semiautomated microinjector and micromanipulator (Eppendorf, Madison, WI) mounted on a Zeiss Axiovert 100M microscope. Vectors expressing chimeric GFP fusion proteins were microinjected at 1 μg/ml in phosphate buffer, pH 7.1, containing TRITC dextran (0.4 mg/ml).

RESULTS AND DISCUSSION

We first established a protocol that preserved both actin tails and vesicles in fixed cells. The fluid phase markers used in live cell studies (10) were unstable during chemical fixation (not shown), so we used fluorescently labeled IgE to tag the vesicles at the heads of actin comet tails. RBL cells express the FcεR1 receptor, and IgE can be prebound to this receptor at the membrane surface and used to follow endocytosis stimulated by PMA (20). Unstimulated cells did not produce rockets, but following stimulation with PMA in hyperosmolar saline (HOS), macropinosomes and actin rockets were visualized using IgE conjugated to Alexa 568 and co-staining with FITC-phalloidin (Fig. 1A). To confirm that all rocket tails were nucleated at the surface of endosomes, a number of IgE-A568-labeled, -stimulated, -fixed, and FITC-phalloidin-stained cells were imaged using confocal microscopy. Of 80 rockets seen in 50 cells, 98% were associated with clearly labeled endosomes, consistent with our previous findings in living cells (10).

Fig. 1
Rockets are composed of F-actin and are nucleated at the surface of endosomes

We previously showed that when used alone, mild HOS and PMA weakly stimulate macropinocytic rocketing but that the frequency of rocketing is enhanced when the agonists are used in combination (10), implying the activation of either two distinct signaling pathways or the synergistic activation of a single pathway. To test the possible involvement of proteinty-rosine phosphorylation/dephosphorylation, we quantified the induction of actin rockets in response to Perv, alone and in combination with HOS and PMA (Fig. 1B). Specifically, induction of rocketing pinosomes with either PMA or Perv was strictly dependent on co-stimulation with HOS, yet a combination of PMA and Perv bypassed this requirement. Although both PMA and Perv stimulated the production of macropinocytic rockets, they are unlikely to act in the same way, the former probably acting through a protein kinase C-dependent pathway and the latter via protein tyrosine phosphorylation. This notion is supported by studies showing protein kinase C localized to rocketing endosomes in activated Xenopus eggs (21). These two signaling routes may be convergent, but both are clearly required to induce actin-based rocketing. The low level induction of actin-based rocketing in the presence of HOS alone suggests simultaneous but weak activation of both pathways. The fact that Perv stimulates vesicle rocketing implicates tyrosine phosphorylation in this phenomenon. Indeed, it has been shown that tyrosine phosphorylation is required for actin-based propulsion of Vaccinia, although not for Listeria or Shigella (22). Despite the potent stimulatory effects of Perv on macropinocytic rocketing in vivo, we did not observe phospho-tyrosine immunoreactivity associated with pinosomes or actin tails in fixed cells (results not shown), suggesting that different types of actin-based rocketing have different requirements for tyrosine phosphorylation. Despite the generation of new pinosomes under these conditions, cellular pinocytosis in the presence of HOS/PMA was significantly lower than with PMA alone (Fig. 1C). Thus, macropinocytic rocketing does not reflect stimulation of pinocytosis per se and is most likely due to the exaggeration of a step in pinocytosis not normally visible (10). Perv alone did not stimulate rocketing but instead induced a dramatic redistribution and polarization of F-actin (Fig. 1D). Other changes in cellular morphology elicited by these agonists are shown in Fig. 1, EH.

These results are consistent with in vitro studies showing that vesicle rocketing is also strongly stimulated by Perv (23, 24). These studies, describing actin-based rocketing of both unidentified vesicles and synthetic vesicles in Xenopus egg extracts, revealed that rocketing in vitro was strongly stimulated by Perv and GTPγS and had an absolute requirement for the small GTP-binding protein Cdc42. The same studies also showed that the membrane composition of synthetic vesicles modulated their recruitment of actin and organization of actin tails. Specifically, it was found that incorporation of PtdIns-4,5P2 or PtdIns-3,4,5P3 into synthetic vesicle membranes relieved the requirement for GTPγS to promote rocketing (23). This is consistent with other studies reporting that co-stimulation of HEK-293 cells with Perv and HOS leads to a marked accumulation of PtdIns-3,4,5P3 through the activation of PtdIns3-kinase(s) (25) and that PtdIns3-kinase has a role in cup closure during phagocytosis and macropinocytosis (26). Despite the observation that vesicle rocketing in vitro was insensitive to wortmannin, the demonstration in those studies of a requirement for phosphoinositides is consistent with a role for PtdIns3-kinase. This prompted us to ask whether or not rocketing in vivo required PtdIns3-kinase(s). Incubation of RBL cells with wortmannin or LY294002 during stimulation with HOS/Perv resulted in inhibition of actin tail formation (Fig. 2, A and B), confirming the requirement for PtdIns3-kinase(s) and supporting the idea that phosphoinositides are involved in macropinocytic rocketing.

Fig. 2
Macropinocytic rocketing is a stress response

To gain further insight into the signaling pathway that regulates macropinocytic rocketing, we tested a variety of other compounds for their ability to block the appearance of F-actin rockets (Table I). Because HOS is known to activate p38 MAP kinase in various cells (2729) and components of this pathway are involved in actin remodeling, we evaluated inhibitors of this pathway. Inhibitors were considered not to have had a specific effect on rocketing unless they met two criteria. First, cells showed complete abolition of macropinosome rocketing, and second, there was no obvious cell death or gross effects on cell morphology that might have indirectly affected macropinocytosis. In addition to LY294002 and wortmannin, SB202190 (Fig. 2C), latrunculin B, and BAPTA-AM all abolished macropinocytic rocketing. Inhibition of rocketing by SB202190 indicates the involvement of p38 MAP kinase, perhaps acting through the downstream effector Hsp27 (30), and is consistent with the appearance of phospho-p38 in RBL cells induced to rocket (Fig. 2C). Cortical actin and membrane ruffling in resting RBL cells treated with wortmannin appeared little different from controls (Fig. 2D), indicating that inhibition of phosphatidylinositol 3-kinase did not block recruitment of F-actin to the plasma membrane. However, RBL cells stimulated to produce rockets in the presence of wortmannin failed to do so, although the cells contained vesicles encircled by F-actin (Fig. 2E). These appear to be macropinosomes that budded from the plasma membrane but which then failed to rocket. Treatment of RBL cells with BAPTA-AM also abolished macropinocytic rocketing (Fig. 2, F and G), revealing the Ca2+ dependence of this activity. Indeed, there are similarities in cells treated with wortmannin and BAPTA-AM. Rather than the characteristic F-actin rockets, cells contained numerous F-actin-decorated vesicles, which tended to be clustered close to the plasma membrane. These experiments establish the following. First, macropinocytic rocketing is dependent on actin polymerization. Second, macropinocytic rocketing requires the production of phosphoinositides. Third, conditions that lead to rocketing also activate and require p38 MAP kinase, and fourth, rocketing is Ca2+-dependent.

Table I
The pharmacology of macropinocytic rocketing

Since vesicle rocketing in vitro can be stimulated by both PtdIns-3,4,5P3 and PtdIns-4,5P2 (23), and given that there may be interconversion between these lipids by kinases and phosphatases, we hypothesized that annexin 2 may bind to these or other phosphoinositides on endocytic vesicles. To obtain a broad view of phosphoinositide binding by annexin 2, we performed overlay blots using lipids immobilized on strips of nitrocellulose (Fig. 3A). Under these conditions, annexin 2 bound to most of the phosphoinositides tested, but binding was only observed in the presence of 50 μm Ca2+. In contrast, annexin 5 failed to bind to any of the immobilized lipids. Unexpectedly, neither annexin 2 nor annexin 5 bound to phosphatidylserine under these conditions. This may be due to the low Ca2+ concentration used in these experiments or to poor accessibility of the phospholipid headgroups in the planar array of the immobilized lipids. To examine phosphoinositide binding by annexin 2 in a more authentic model membrane, we performed liposome sedimentation studies in which annexin 2 was added to phospholipid vesicles composed of DOPC containing a 2.5% molar fraction of PtdIns3P, PtdIns4P, PtdIns-3,4P2, or PtdIns-4,5P2, in the presence and absence of Ca2+ (Fig. 3, B and C). The results show that annexin 2 bound only to liposomes containing PtdIns-4,5P2, and binding occurred in both the presence and the absence of 50 μm Ca2+. In contrast, annexin 5 failed to bind to any of the lipid mixes (Fig. 3, D and E).

Fig. 3
Annexin 2 binds to liposomes containing PtdIns-4,5P2

Next we prepared DOPC liposomes containing a range of concentrations of PtdIns-4,5P2 to assess the affinity of annexin 2 for this phospholipid (Fig. 3F). The results show that at low micromolar concentrations of PtdIns-4,5P2, annexin 2 has an absolute requirement for Ca2+ for binding, but that at concentrations of PtdIns-4,5P2 greater than 10 μm, annexin 2 binds independently of Ca2+. Quantitative densitometric scanning of Coomassie-stained gels revealed that in the presence of 50 μm Ca2+, binding of annexin 2 to PtdIns-4,5P2-containing liposomes is half-maximal at ~5 μm PtdIns-4,5P2. To put these findings in a broader context, half-maximal binding to PtdIns-4,5P2 has been reported in the range 1–300 μm for various pleckstrin homology domains (17, 31), whereas other PtdIns-4,5P2-binding motifs, such as the ANTH (AP180 N-terminal homology) domain (13), bind with a similar affinity to annexin 2 but lack the high specificity of annexin 2 for PtdIns-4,5P2. Thus, the affinity of annexin 2 for PtdIns-4,5P2 is comparable with those reported for diverse PtdIns-4,5P2-binding domains, but the selectivity of annexin 2 for PtdIns-4,5P2 versus other phosphoinositides is high as compared with many PtdIns-4,5P2-binding proteins. Significantly, the affinity of annexin 2 for PtdIns-4,5P2 was increased in liposomes containing approximately physiological concentrations of phosphatidylserine (Fig. 3G), suggesting that in biological membranes, there may be cooperativity of binding. In this experiment we also compared annexin 2 with annexin 1, its closest relative in molecular phylogenetic terms and therefore the most likely among other annexins to exhibit binding to PtdIns-4,5P2. The results show that annexin 1 indeed displays similar characteristics to annexin 2, binding with high affinity to liposomes containing a mixture of PtdIns-4,5P2 and phosphatidylserine and with lower affinity to either lipid alone. Taken together, the results in Fig. 1 demonstrate that annexin 2 specifically binds PtdIns-4,5P2 and not to other polyphosphoinositides and show that the ability to bind this lipid is not a generic annexin property but is restricted to a subgroup within that family that also includes annexin 1.

There is increasing evidence that phosphoinositide lipids function at least in the initial recruitment and assembly of actin at sites of vesicle formation (15, 32) and that actin-dependent transport is responsible for the subsequent inward movement of endocytic, pinocytic, and phagocytic vesicles (11, 16, 33, 34). We therefore investigated whether or not PtdIns-4,5P2 is enriched in motile macropinosomes or at sites of pinosome formation in the RBL experimental model. In RBL cells induced to generate rocketing macropinosomes, then fixed and processed for indirect immunofluorescence, we observed modest co-localization of F-actin and PtdIns-4,5P2 (Fig. 4A). More striking association of PtdIns-4,5P2 with pinosomes was observed in fixed cells preloaded with TMR-PtdIns-4,5P2. In performing z-sections of fixed RBL cells, we consistently observed concentration of PtdIns-4,5P2 immunoreactivity or TMR-PtdIns-4,5P2 at the apical cell surface (Fig. 4B). This is significant because the apical cell surface is the site at which pino-some formation is most concentrated. Thus, a z-section image of a live RBL cell expressing actin-GFP shows the pinosome at the basal cell surface, with the trailing actin tail extending upwards to the apical surface (Fig. 4B). Consistent with these findings, we also observed co-localization of annexin 2-GFP-and Cy3-labeled anti-PtdIns-4,5P2 antibody at the apical surface of activated RBL cells (Fig. 4C). These results, obtained by a number of different approaches to visualize PtdIns-4,5P2, demonstrate that the sites of pinosome formation in activated RBL cells are enriched in PtdIns-4,5P2, annexin 2, and actin.

Fig. 4
PtdIns-4,5P2 is enriched at sites of pinosome formation in RBL cells

To test the possible association between annexin 2 and PtdIns-4,5P2 in a different model, we microinjected Ptk cells with GFP fusions of annexins 2 and 5, together with the constitutively active and inactive mutants of Arf6. Arf6 is a GTPase that regulates endosomal trafficking (15, 16) and activates phosphatidylinositol 4-phosphate 5 kinase, resulting in elevated levels of PtdIns-4,5P2 at the plasma membrane and on intracellular vesicles (35). Insight into the phosphoinositide lipid signaling pathway has come from studies using the Q67L constitutively active and T27N inactive mutants of Arf6 (15, 16). In Ptk cells, annexin 2-GFP appeared localized throughout the cytoplasm but excluded from the nucleus, whereas annexin 5-GFP and GFP were present in both the cytosol and the nucleus (Fig. 5A). Co-injection of expression plasmids encoding the active Q67L mutant of Arf6 together with either GFP or annexin 5-GFP had no effect on the localization of either fusion protein (Fig. 5B). However, co-injection of Q67LArf6 with an-nexin 2-GFP led to a marked redistribution of annexin 2-GFP to the plasma membrane and to intracellular structures that, given the known activities of Arf6 and annexin 2, are most likely to be components of the endocytic pathway (Fig. 5C). In contrast, when co-injected with T27NArf6, annexin 2-GFP remained mostly cytosolic, but with a focal enrichment in the perinuclear region. Accumulation of perinuclear Arf6-positive vesicles has been reported previously in cells expressing the T27N mutant (16), and annexin 2 becomes concentrated in a perinuclear compartment in PC12 cells in which endocytic vesicle traffic is blocked.2 Thus, accumulation of annexin 2 in perinuclear vesicles may be a feature of cells in which normal endocytic processing is perturbed.

Fig. 5
Regulation of annexin 2 localization by Arf6

Taken together, these results are consistent with those presented for annexins 2 and 5 in Fig. 3. Thus, annexin 5 does not bind to liposomes containing PtdIns-4,5P2 in vitro, nor does it respond to the activating Q67L mutant of Arf6 in vivo, whereas annexin 2 binds to PtdIns-4,5P2 in vitro and exhibits a response to Q67LArf6 that would be expected for a PtdIns-4,5P2-binding protein in vivo. In conclusion, we have shown that actin-based macropinosome motility is regulated by the stress-response pathway and that rocketing macropinosomes contain PtdIns-4,5P2. We have identified a new phospholipid binding specificity for annexin 2, which unlike binding to phosphatidylethanolamine and phosphatidylserine, does not appear to be a generic annexin property. The affinity of annexin 2 for PtdIns-4,5P2 is comparable with that of many other PtdIns-4,5P2-binding proteins, and the results of studies in RBL and Ptk cells are consistent with a direct interaction between annexin 2 and PtdIns-4,5P2 in vivo. These findings provide further evidence that annexin 2 is involved in the dynamic remodeling of the membrane and actin cytoskeleton that depends on PtdIns-4,5P2 and occurs during the formation of endocytic vesicles.

Acknowledgments

We are grateful to Volker Gerke for the regular supplies of HH7 antibody and to Mark Shipman, William Hinkes, and Keith Morris for help with the confocal microscopy.

Footnotes

*This work was supported by the Wellcome Trust, Medical Research Council, Fight for Sight and the European Commission (contract number BIO4CT960083).

1The abbreviations used are: RBL, rat basophilic leukemia; PtdIns3P, phosphatidylinositol 3-phosphate; PtdIns4P, phosphatidylinositol 4-phosphate; PtdIns-4,5P2, phosphatidylinositol 4,5-bisphosphate; PtdIns-3,4P2, phosphatidylinositol 3,4-bisphosphate; MAP, mitogen-activated protein; GFP, green fluorescent protein; TMR, tetramethylrhodamine; TRITC, tetramethylrhodamine isothiocyanate; HOS, hyperosmolar saline; Perv, pervanadate; GTPγS, guanosine 5′-3-O-(thio)triphosphate; PMA, phorbol myristic acid; PBS, phosphate-buffered saline; HBS, HEPES-buffered saline; FITC, fluorescein isothiocyanate; DOPC, dioleoylphosphatidylcholine; BB, Dioleoylphosphatidylcholine.

2C. J. Merrifield and S. E. Moss, unpublished observations.

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