Actin-binding proteins that coordinate contractile ring assembly in fission yeast include IQGAP (Rng2p), type-II myosins (Myo2p and Myp2p), Arp2/3 complex (arp3 and wsp1), tropomyosin (Cdc8p), profilin (Cdc3p), and the formin Cdc12p (for reviews see Le Goff et al., 1999 Feierbach and Chang, 2001a Guertin et al., 2002). Both preexisting and newly polymerized actin filaments contribute to the contractile ring, which turns over throughout cleavage ( Pelham and Chang, 2002). The fission yeast Schizosaccharomyces pombe is the cell with the most complete inventory of cytokinesis genes, >50 ( Le Goff et al., 1999 Guertin et al., 2002). Lacking biochemical assays for cytokinesis, the field has depended largely on genetics to identify the participating proteins. These properties explain why contractile ring assembly requires both formin and profilin and why viability depends on the ability of profilin to bind both actin and poly- l-proline.Ĭompared with the rapidly accumulating evidence about the assembly of the actin network at the leading edge of cells ( Pollard and Borisy, 2003), much less is known about the assembly of the actomyosin contractile ring during cytokinesis ( Robinson and Spudich, 2000). Thus, formins are profilin-gated barbed end capping proteins with the ability to initiate actin filaments from actin monomers bound to profilin. On the other hand, Cdc12(FH1FH2)p blocks annealing even in the presence of profilin. Profilins that bind both actin and poly- l-proline inhibit nucleation by Cdc12(FH1FH2)p, but polymerization of monomeric actin is faster, because the filaments grow from their barbed ends at the same rate as uncapped filaments.
The maximum yield is one filament pointed end per six formin polypeptides. Like capping protein, purified Cdc12(FH1FH2)p caps the barbed end of actin filaments, preventing subunit addition and dissociation, inhibits end to end annealing of filaments, and nucleates filaments that grow exclusively from their pointed ends. Cells overexpressing Cdc12(FH1FH2)p stop growing with excessive actin cables but no contractile rings. Expression of a construct consisting of the Cdc12 FH1 and FH2 domains complements a conditional mutant of Cdc12 at the restrictive temperature, but arrests cells at the permissive temperature. The proline-rich formin homology (FH) 1 domain binds profilin, and the FH2 domain binds actin. The fission yeast Schizosaccharomyces pombe requires the formin Cdc12p and profilin (Cdc3p) early in the assembly of the contractile ring. Cytokinesis in most eukaryotes requires the assembly and contraction of a ring of actin filaments and myosin II.
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