ISOLATION AND CHARACTERIZATION OF FISSION YEAST MUTANTS DEFECTIVE IN THE ASSEMBLY AND PLACEMENT OF THE CONTRACTILE ACTIN RING

Fission yeast cells divide by medial cleavage using an actin-based con tractile ring, We have conducted a genetic screen for temperature-sens itive mutants defective in the assembly and placement of this actin ri ng, Six genes necessary for actin ring formation and one gene necessar y for placement of the actin ring have now been identified, The genes can be further organized into different phenotypic groups, suggesting that the gene products may have different functions in actin ring form ation, Mutants of cdc3 and cdc8, which encode profilin and tropomyosin respectively, display disorganized actin patches in all cells, cdc12 and cdc15 mutants display disorganized actin patches during mitosis, b ut normal interphase actin patterns, cdc4 and rng2 mutants display dis organized actin cables during mitosis, but normal interphase actin pat terns, In midi mutants, the actin ring and septum are positioned at ra ndom locations and angles on the cell surface, although the nucleus is positioned normally, indicating that the mini gene product is require d to couple the division site to the position of the nucleus, mid1 mut ant cells may reveal a new cell cycle checkpoint in telophase that coo rdinates cell division and the proper distribution of nuclei, The acti n ring forms medially in a beta-tubulin mutant, showing that actin rin g formation and placement are not dependent on the mitotic spindle,