Ssp1 promotes actin depolymerization and is involved in stress response and new end take-off control in fission yeast

The ssp1 gene encodes a protein kinase involved in alteration of cell polar ity in Schizosaccharomyces pombe. ssp1 deletion causes stress sensitivity, reminiscent of defects in the stress-activated MAP kinase, Spc1; however, t he two protein kinases do not act through the same pathway. Ssp1 is localiz ed mainly in the cytoplasm, but after a rise in external osmolarity it is r apidly recruited to the plasma membrane, preferentially to active growth zo nes and septa. Loss of Ssp1 function inhibits actin relocalization during o smotic stress, in cdc3 and cdc8 mutant backgrounds, and in the presence of latrunculin A, implicating Ssp1 in promotion of actin depolymerization. We propose a model in which Ssp1 can be activated independently of Spc1 and ca n partially compensate for its loss. The ssp1 deletion mutant exhibited mon opolar actin distribution, but new end take-off (NETO) could be induced in these cells by exposure to KCl or to latrunculin A pulse treatment. This tr eatment induced NETO in cdc10 cells arrested in G1 but not in tea1 cells. T his suggests that cells that contain intact cell end markers are competent to undergo NETO throughout interphase, and Ssp1 is involved in generating t he NETO stimulus by enlarging the actin monomer pool.