Phosphoinositides are phosphorylated derivatives of PtdIns. The chemical structures of PtdIns and Phosphoinositides are shown highlighting the inositol headgroup, glycerol backbone and two fatty acyl chains. The inositol headgroup can be combinatorially phosphorylated at the D-3OH, -4OH, -5OH positions of the inositol ring as indicated in red.

Phosphoinositide metabolism in mammalian cells. The execution points of the mammalian PtdIns kinases and phosphoinositide phosphatases that regulate the synthesis and turnover of Phosphoinositides, respectively, are identified.

Domain organization of phosphoinositide phosphatases. The domain structures of PTEN, MTM1, Sac1, Fig4, OCRL1, INPP5B, Synaptojanin 1,2, and SHIP1,2 are illustrated. Relevant functional domains and motifs are indicated.

Dynamic regulation of Sac1 localization in yeast and mammals. When yeast cells are in exponential growth, the ER dolicholphosphate mannose synthase Dpm1p interacts with yeast Sac1 (ySac1) via their respective trans-membrane domains. This interaction restricts Sac1 to the ER, results in elevated Golgi PtdIns-4-P, and promotes robust secretory activity (A). Upon nutrient limitation, the ySac1 interaction with Dpm1p is broken and ySac1 escapes to Golgi membranes. Increased Golgi ySac1 reduces PtdIns-4-P levels and, subsequently, secretory activity (B). In mammalian cells, growth factor signaling promotes retrograde transport of Sac1 from Golgi membranes to the ER via an ARF- and COPI-dependent pathway. Consequent elevation of Golgi PtdIns-4-P promotes optimal secretion (C). In quiescent cells, Sac1 oligomerizes and translocates to the Golgi via a COPII-mediated pathway. Increased Golgi Sac1 reduces PtdIn-4-P pools and downregulates secretory activity (D).

Phosphoinositide metabolism in the yeast Saccharomyces cerevisiae. The execution points of the yeast PtdIns kinases and phosphoinositide phosphatases that regulate the synthesis and turnover of phosphoinositides, respectively, are identified.

Catalytic mechanisms of phosphoinositide phosphatases. (A) The CX₅R(T/S) phosphoinositide phosphatases of the PTP superfamily. These enzymes catalyze a double-displacement reaction where the leaving group alcohol is displaced by an active-site nucleophile. The resulting phospho-enzyme intermediate is resolved by transfer of the PO₃ group to an acceptor water molecule. An aspartic acid subsequently donates a proton to the leaving-group oxygen to regenerate an uncharged hydroxy group at the position from which the PO₃ group was displaced (Guan and Dixon, 1991; Fauman and Saper, 1996; Hughes et al., 2000a). (B) The inositol polyphosphate 5’-phosphatases of the AP endonuclease superfamily. These enzymes catalyze a displacement reaction where the leaving group alcohol is displaced by an actived water molecule as nucleophile. An invariant Asp residue, held in properly position by H-bonding with a conserved Asn, activates the nucleophilic water (1). A His/Asp pair cooperates with an invariant Asn to position the target phosphate bond for hydrolysis. Mg²⁺ is thought to stabilize a transition state in the reaction. There is no phosphoenzyme intermediate in this catalytic mechanism as the PO₃ group is transferred directly to the nucleophilic water molecule (2). Either water, or some other functional group of the enzyme (both possibilities generically designated as XH*; 3), donates a proton to the leaving-group oxygen to regenerate an uncharged hydroxyl group at the position from which the PO₃ group was displaced.

Phosphoinositide phosphatases and regulation of membrane trafficking. The phosphoinositide 4-phosphatase Sac1 is localized primarily in ER and Golgi membranes. OCRL1 (yellow) is localized to the Golgi/endosomal system and is a cargo of the clathrin-coated vesicles (CCVs) responsible for bidirectional membrane trafficking between TGN and endosomes. In the neuronal presynaptic plasma membrane, a PtdIns- 4,5-P₂ pool is accessed by synaptojanin 1 during CCV uncoating. In non-neuronal cells, synaptojanin 2 (maroon) functions at an early stage of clathrin-mediated endocytosis. MTM1 (pink) regulates endocytic traffic and PTEN (red) degrades a PtdIns-3,4,5-P₃ pool involved in membrane trafficking events associated with phagocytosis. Abbreviations: CCP, clathrin-coated pit; CCV, clathrin-coated vesicle; EE, early endosome; ER, endoplasmic reticulum; Ly, lysosome; MVB/LE, multivesicular body/late endosome; PGC, post-Golgi carrier; SV, secretory vesicle; TGN, trans-Golgi network.