Hof1 and Chs4 interact via F-BAR domain and Sel1-like repeats to control extracellular matrix deposition during cytokinesis in budding yeast

Localized extracellular matrix (ECM) remodeling, which mainly involves synthesis and breakdown of specific polysaccharides, is important for cytokinesis and cell separation in most, if not all, organisms [1-11]. This remodeling event is thought to stabilize the cleavage furrow and maintain cell shape. The remodeling requires exocytic delivery to and endocytic removal from the division site of synthetic and hydrolytic enzymes [12-20] that is spatiotemporally coordinated with a cytoskeletal structure pertaining to a kingdom of life, for example, the FtsZ ring in bacteria [21, 22], the phragmoplast in plants [23, 24], and the actomyosin ring (AMR) in fungi and animals [25-27]. While the cytoskeletal structures have been analyzed extensively in different systems, the ECM remodeling and its interplay with the cytoskeletal partner remains poorly understood. In the budding yeast Saccharomyces cerevisiae, the ECM remodeling refers to the collective activities of chitin synthase-II (CSII, Chs2)-mediated primary septum (PS) formation that closely follows AMR constriction, glucan synthase (Fks1 and Fks2) and chitin synthase-III (CSIII, the catalytic subunit Chs3 and its activator Chs4)-mediated secondary septum (SS) formation at both sides of the PS near or after the end of AMR constriction, and, finally, glucanase and endochitinase-mediated degradation of the PS and some SS, leading to cell separation [27, 28]. Surprisingly, both Chs2 and Chs3 are delivered to the division site at the onset of cytokinesis [14, 29, 30]. What keeps CSIII inactive until SS formation remains unknown. Here, we show that Hof1 [31-33], via its F-BAR domain [34], competes with Chs3 for binding to the Sel1-like repeats (SLR) of Chs4 and inhibits Chs3-mediated chitin synthesis during cytokinesis. In addition, Hof1 is required for the rapid accumulation as well as efficient endocytic removal of Chs4 at the division site. Thus, this study uncovers a dual mechanism by which Hof1 controls timely activation of Chs3 during cytokinesis, and defines a novel interaction and function between the conserved F-BAR domain and SLR that are otherwise known for their abilities to bind membrane lipids [35-37] and scaffold protein complex formation [38].