Role of the Hofl-Cyk3 interaction in cleavage-furrow ingression and primary-septum formation during yeast cytokinesis

In Saccharomyces cerevisiae, it is well established that Hofl, Cyk3, and Inn1 contribute to septum formation and cytokinesis. Because hofl Delta and cyk3 Delta single mutants have relatively mild defects but hofl Delta cyk3 Delta double mutants are nearly dead, it has been hypothesized that these proteins contribute to parallel pathways. However, there is also evidence that they interact physically. In this study, we examined this interaction and its functional significance in detail. Our data indicate that the interaction 1) is mediated by a direct binding of the Hofl SH3 domain to a proline-rich motif in Cyk3; 2) occurs specifically at the time of cytokinesis but is independent of the (hyper) phosphorylation of both proteins that occurs at about the same time; 3) is dispensable for the normal localization of both proteins; 4) is essential for normal primary-septum formation and a normal rate of cleavage-furrow ingression; and 5) becomes critical for growth when either Inni or the type II myosin Myol (a key component of the contractile actomyosin ring) is absent. The similarity in phenotype between cyk3 Delta mutants and mutants specifically lacking the Hofl-Cyk3 interaction suggests that the interaction is particularly important for Cyk3 function, but it may be important for Hofl function as well.