Fission yeast Rng3p: an UCS-domain protein that mediates myosin II assembly during cytokinesis

Cell division in many eukaryotes, including the fission yeast Schizosacchar omyces pombe, utilizes a contractile actomyosin ring. In S. pombe, the acto myosin ring is assembled at the medial cortex: upon entry into mitosis and constricts at the end of anaphase to guide the centripetal deposition of th e septum, Despite identification of several structural components essential for actomyosin ring assembly, the interdependencies between these gene-pro ducts in the process of ring assembly are unknown. This study investigates the role of Rng3p, a member of the UCS-domain containing protein family ((U ) under bar nc-45p, (C) under bar ro1p, (S) under bar he4p), in actomyosin ring assembly. Null mutants in rng3 resemble deletion mutants in the type I I myosin heavy chain (myo2) and rng3(ts) mutants show strong negative inter actions with the myo2-E1 mutant, suggesting that Rng3p is involved in modul ating aspects of type II myosin function. Interestingly, a green fluorescen t protein (GFP) tagged Rng3p fusion is detected at the division site in the myo2-E1 mutant, but not in other myo2-alleles, wild-type cells or in 18 ot her cytokinesis mutants. Assembly and maintenance of Rng3p at the division site in the myo2-E1 mutant requires F-actin. Rng3p is also required for the proper assembly of Myo2p and F-actin into a functional actomyosin ring but is not necessary for their accumulation at the division site. We conclude that Rng3p is a novel component of the F-actin cytoskeleton essential for a late step in actomyosin ring assembly and that it might monitor some aspec t of type II myosin assembly during actomyosin ring construction.