(A) The three motors used: monomeric Kar3MD, Kar3Cik1 heterodimer, and dimeric D. melanogaster Ncd MC1.
(B) Coomassie-stained gel (2 M Urea, 8% SDS-PAGE) and Western blot showing protein purity and antibody specificity.
(C) Analytical gel filtration of Kar3Cik1, dimeric conventional kinesin K401, and Kar3MD with M_r based on amino acid sequence. Stokes radius of Kar3Cik1 = 4.0 nm, K401 = 3.8 nm, and Kar3MD = 3.4 nm.
(D) Kar3Cik1 minus-end-directed MT motility in the presence of MgATP. Arrowheads denote the bright MT minus end, and the asterisks, the dim MT plus end. The scale bar represents 5 μm.
(E–G) The steady-state ATPase kinetics of Kar3Cik1 as a function of MTs (E) and MgATP (F and G). Both (E) and (F) were performed at 25 μM Taxol, at which the MTs were stable and resistant to Kar3Cik1-promoted depolymerization. (E) Final concentrations: 1 μM Kar3Cik1, 1–40 μM tubulin, 25 μM Taxol, and 1 mM Mg[α³²P]ATP. Steady-state parameters: k_cat = 0.35 ± 0.02 s⁻¹; K_1/2,Mt = 6.3 ± 1.3 μM. Inset: Kar3Cik1 was incubated with increasing concentrations of tubulin as MTs (black), soluble tubulin heterodimer (red), or in the absence of tubulin (green circle), and the reactions were initiated with MgATP. Final concentrations: 0.5 μM Kar3Cik1, 0–3 μM tubulin, ± 3 μM Taxol, and 100 μM Mg[α³²P]ATP. The ATPase rate in the absence of tubulin (green) was 0.014 ± 0.006 s⁻¹; soluble tubulin (red) = 0.015 ± 0.002 s⁻¹. (F) Steady-state ATPase at stable MT conditions. Final concentrations: 1 μM Kar3Cik1, 40 μM tubulin, 25 μM Taxol, and 1–700 μM Mg[α³²P]ATP. Kar3Cik1: k_cat = 0.37 ± 0.005 s⁻¹; K_m,ATP = 8.2 ± 0.53 μM. Inset: Initial phase at 1–200 μM MgATP. (G) The steady-state ATPase at MT depolymerizing conditions. Final concentrations: 1 μM Kar3Cik1, 40 μM tubulin, 15 μM Taxol, and 1 mM Mg[α³²P]ATP. Kar3Cik1: k_cat = 0.26 ± 0.017 s⁻¹; K_{m, ATP} = 71.2 ± 16.2 μM. The table shows the steady-state parameters of Kar3Cik1 in comparison to Kar3MD [29], and dimeric Ncd MC1 [30]. “ND” denotes “not determined.”

MT•motor complexes were formed in the presence of MgAMPPNP and imaged at t = 0. MT depolymerization was initiated by MgATP plus an ATP regeneration system.
(C, F, and I) Merge of times 0 and 60 min to show MT shortening (yellow) in comparison to the original length (red).
(C) The MTs exhibited shortening from only one end in the presence of Kar3Cik1 plus 1.5 mM MgATP. The scale bar represents 5 μm.
(F) MT shortening from the plus end (*) of the MT in the presence of Kar3Cik1 and 1.5 mM MgATP. The scale bar represents 5 μm. (D)–(R) are at the same magnification.
(G–I) MT shortening from the plus end (*) of MTs in the presence of dimeric Ncd and 1.5 mM MgATP; the MT seed (arrowhead) denotes the MT minus end.
(J–O) MT shortening from both ends promoted by Xenopus MCAK.
(P–R) Conventional kinesin K401 did not promote MT shortening. The MT shortening rate is reported as nm/min ± the SEM.

(1) Mating-specific pheromones induce Kar3 and Cik1 expression as well as the formation of the rounded cell protrusions or “shmoos.” (2 and 3) The astral MTs are highly dynamic, and the MT plus ends become associated with the cortex. (4) For MT shortening, Kar3Cik1 motors (green spheres) are targeted to the MT plus ends. Kar3Cik1-promoted MT depolymerization shortens the MTs to pull each nucleus into its shmoo. (5) Localized cell-wall breakdown allows cytoplasm fusion, resulting in interdigitating MTs of opposite polarity. (6) Kar3Cik1 can now use its minus-end-directed force for both MT sliding and MT shortening to form the diploid nucleus.

(A and B) Representative gel slices above each set of bars show tubulin partitioning to either the supernatant (S) or to the MT pellet (P). Each bar is the mean of three replicates, and the error bars report the SEM. MTs in the presence (cyan bars) or absence (blue bars) of Kar3Cik1 were incubated with different nucleotides and MT stabilizing agents to determine the substrates for Kar3Cik1 depolymerization. Final concentrations: ± 50 nM Kar3Cik1, 500 nM tubulin, 3 or 20 μM Taxol or 1 mM MgGMPcPP, and 1 mM MgATP or 1 mM MgAMPPNP. (B and C) MTs stabilized with 3 μM Taxol were incubated with Kar3Cik1 in the presence of 1 mM MgATP. The reactions were terminated at specific times with 2 mM MgAMPPNP, and then centrifugation and analysis by SDS-PAGE were performed. The percent of tubulin partitioning to the supernatant was plotted as a function of time, and the data were fit to a single exponential function with k_obs = 0.07 ± 0.01 s⁻¹

Mt•motor complexes were preformed in solution in the presence of MgAMPPNP, and glutaraldehyde fixation and centrifugation through a glycerol cushion onto coverslips followed. The coverslips were processed for immunofluorescence. Final concentrations: 50 nM or 100 nM motor, 500 nM tubulin, 3 μM Taxol, and 1 mM MgAMPPNP. All panels represent the same magnification. The scale bar represents 5 μm. The MT seed (arrowhead) marks the MT minus end, and (*) denotes the MT plus end extended from the seed.
(A, D, G, J, and M) Rhodamine-labeled tubulin.
(B, E, H, K, and N) Anti-Kar3/Kinesin fluorescence.
(C, F, I, L, and O) Merge of the two channels to show MT-motor colocalization. The table shows the summary of MT localization events scored for the four motors and the no-motor control.