HPV16 E7 phosphorylation in fission yeast: characterization and biological effects.

Human papillomavirus type 16 E7 protein (HPV-16 E7), synthesised in Schizosaccharomyces pombe, is both phosphorylated and targeted to the nucleus [Tommasino et al., Gene 93 (1990) 265-270] as is E7 protein synthesized in primate cells. Further analysis of E7 expression in fission yeast indicates that: (i) E7 protein synthesised in S. pombe is phosphorylated only on the Ser residues which are part of a casein kinase II consensus site, as it has been shown to be the cause in human cells, and is tightly associated with the nuclear matrix; (ii) synthesis of wild type, phosphorylated E7 is responsible for a significant increase in S. pombe doubling time; and (iii) E7 phosphorylation is not required for the tight association of the protein with the nuclear matrix, but E7 mutants, in which one (Ser31) or both phosphorylated serines (Ser31 and Ser32) have been substituted, lack any effect on cell-cycle duration.