Figure 7.

Model for actin polymerization by Las17 and Ysc84. Our data indicate that in cells, Las17 is able to bind two actin monomers—one monomer at its previously recognized WH2, actin-binding domain and one monomer at the region defined in this study encompassed by amino acids 300-422. When Ysc84-Las17 and actin associate in a ternary complex, Ysc84 is able to bind to and stabilize the actin dimer presented by Las17 and thereby promote polymerization. Ysc84 can then remain associated with actin and in this way actin becomes bundled. In the absence of Las17, Ysc84 is not able to interact with actin most likely because its own SH3 domain sterically blocks binding.