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Published 9 June 2003. doi:10.1083/jcb.200211078
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© The Rockefeller University Press, 0021-9525/2003/6/875 $5.00
The Journal of Cell Biology, Volume 161, Number 5, 875-887

Article

 The fission yeast cytokinesis formin Cdc12p is a barbed end actin filament capping protein gated by profilin

David R. Kovar, Jeffrey R. Kuhn, Andrea L. Tichy and Thomas D. Pollard

Department of Molecular, Cellular, and Developmental Biology, Yale University, New Haven, CT 06520

Address correspondence to Thomas D. Pollard, MCDB, KBT-548, Yale University, P.O. Box 208103, New Haven, CT 06520-8103. Tel.: (203) 432-3460. Fax: (203) 432-6161. E-mail: thomas.pollard{at}yale.edu

Cytokinesis in most eukaryotes requires the assembly and contraction of a ring of actin filaments and myosin II. The fission yeast Schizosaccharomyces pombe requires the formin Cdc12p and profilin (Cdc3p) early in the assembly of the contractile ring. The proline-rich formin homology (FH) 1 domain binds profilin, and the FH2 domain binds actin. 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. Cells overexpressing Cdc12(FH1FH2)p stop growing with excessive actin cables but no contractile rings. 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. The maximum yield is one filament pointed end per six formin polypeptides. 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. On the other hand, Cdc12(FH1FH2)p blocks annealing even in the presence of profilin. Thus, formins are profilin-gated barbed end capping proteins with the ability to initiate actin filaments from actin monomers bound to profilin. 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.

Key Words: actin cables; annealing; capping protein; contractile ring; S. pombe

The online version of this article includes supplemental material.

* Abbreviations used in this paper: FH, formin homology; MmCP, Mus musculus capping protein; TIRF, total internal reflection fluorescence.


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