Skp1 and the F-box protein Pof6 are essential for cell separation in fission yeast

Here we report functional characterization of the essential fission yeast Skp1 homologue. We have created a conditional allele of skp1 (skp1-3f) mimicking the mutation in the budding yeast skp1-3 allele. Although budding yeast skp1-3 arrests at the G(1)/S transition, skp1-3f cells progress through S phase and instead display two distinct phenotypes. A fraction of the skp1-3f cells arrest in mitosis with high Cdc2 activity. Other skp1-3f cells as well as the skp1-deleted cells accumulate abnormal thick septa leading to defects in cell separation. Subsequent identification of 16 fission yeast F-box proteins led to identification of the product of pof6 (for pombe F-box) as a Skp1-associated protein. Interestingly, cells deleted for the essential pof6 gene display a similar cell separation defect noted in skp1 mutants, and Pof6 localizes to septa and cell tips. Purification of Pof6 demonstrates association of Skp1, whereas the Pcul cullin was absent from the complex. These findings reveal an essential non-Skp1-Cdc53/Cullin-F-box protein function for the fission yeast Skp1 homologue and the F-box protein Pof6 in cell separation.