Investigating septin-dependent, actomyosin-ring-independent cytokinesis in Saccharomyces cerevisiae

In the budding yeast Saccharomyces cerevisiae, a ring of myosin II forms in a septin-dependent manner at the presumptive budding site in late G1. This ring remains at the bud neck until the onset of cytokinesis, when actin is recruited to it and the resulting actomyosin ring contracts. Concurrently, a septum of cell wall (including a chitinous primary septum synthesized by Chs2p) is formed to complete cytokinesis. Mutants lacking MYO1 (the only myosin II gene) have varyingly severe phenotypes that can be suppressed either by deletion of a nonessential subunit of the anaphase-promoting complex or cyclosome (APC/C) or by overexpression of IQG1 or CYK3. Iqg1p is the only S. cerevisiae IQGAP and is required for both actomyosin-ring formation and primary-septum formation, and hence for normal cytokinesis. Because the actomyosin ring itself is not essential for primary-septum formation, Iqg1p must have another cytokinetic function(s), which may involve stimulating Cyk3p (a septin-dependent protein involved in cytokinesis) and Hof1p (a possible linker between the actomyosin ring and the primary-septum-synthesis machinery) to function in a septin-dependent, actomyosin-ring-independent pathway. Subsequent analyses showed that Iqg1p is increased in abundance and persists after cytokinesis in APC/C mutants, that Iqg1p is ubiquitinated directly by APC/C in vitro, and that a nondegradable Iqg1p (missing a novel recognition motif) can suppress myo1Delta phenotypes. These data suggest that Iqg1p is a direct target of the APC/C. To identify additional proteins involved in septin-dependent, actomyosin-ring-independent cytokinesis, dosage-suppressor screens have been conducted, using myo1Delta cyk3Delta and myo1Delta hof1Delta synthetic-sick mutants. Some of the genes identified include EGT2 (encoding a cell-wall endoglucanase), ECM33 (encoding a GPI-anchored protein that may regulate cell-wall organization), and a truncated CDC24 (encoding the GEF for the Rho-type GTPase Cdc42p) that lacks the C-terminal PB1 domain. Overexpressing Ecm33p or the truncated Cdc24p can also suppress the iqg1Delta and chs2Delta growth defects and restore primary-septum formation in iqg1Delta cells. Furthermore, the suppression by truncated Cdc24p appears to be Cdc42p-independent and may involve another Rho-type GTPase(s). Finally, the dosage-suppressor screen also uncovered a gene, YJL055W, that when overexpressed directly suppresses the inviability of URA3+ cells in the presence of 5-fluoroorotic acid.