Targeting and functional mechanisms of the cytokinesis-related F-BAR protein Hof1 during the cell cycle.

F-BAR proteins are membrane-associated proteins believed to link the plasma membrane to the actin cytoskeleton in cellular processes such as cytokinesis and endocytosis. In the budding yeast Saccharomyces cerevisiae, the F-BAR protein Hof1 localizes to the division site in a complex pattern during the cell cycle and plays an important role in cytokinesis. However, the mechanisms underlying its localization and function are poorly understood. Here we show that Hof1 contains three distinct targeting domains that contribute to cytokinesis differentially. The N-terminal half of Hof1 localizes to the bud neck and the sites of polarized growth during the cell cycle. The neck localization is mediated mainly by an interaction between the second coiled-coil region in the N-terminus and the septin Cdc10, whereas the localization to the sites of polarized growth is mediated entirely by the F-BAR domain. In contrast, the C-terminal half of Hof1 interacts with Myo1, the sole myosin-II heavy chain in budding yeast, and localizes to the bud neck in a Myo1-dependent manner from the onset to the completion of cytokinesis. We also show that the SH3 domain in the C-terminus plays an important role in maintaining the symmetry of Myo1 ring constriction during cytokinesis and that Hof1 interacts with Chs2, a chitin synthase that is required for primary septum formation. Together these data define a mechanism that accounts for the localization of Hof1 during the cell cycle and suggest that Hof1 may function in cytokinesis by coupling actomyosin ring constriction to primary septum formation through interactions with Myo1 and Chs2.