Developmental and cell-cycle regulation of Caenorhabditis elegans HCF phosphorylation

HCF-1 is a mammalian protein required for cell proliferation. It is also in volved in transcriptional activation of herpes-simplex-virus immediate-earl y gene transcription in association with the viral transactivator VP16. HCF -1 and a related protein called HCF-2 possess a homologue in Caenorhabditis elegans that can associate with and activate VP16. Here, we demonstrate de velopmental regulation of C. elegans HCF (CeHCF) phosphorylation: a hyperph osphorylated form of CeHCF is present in embryos, whereas a hypophosphoryla ted form is present in L1 larvae. The phosphorylation patterns of endogenou s CeHCF in worms and ectopically synthesized CeHCF in mammalian cells are r emarkably similar, suggesting that the way CeHCF can be recognized by kinas es is conserved in animals. Phosphorylation-site mapping of endogenous CeHC F, however, revealed that phosphorylation occurs at four clustered sites in the region of the protein that is not highly conserved among HCF proteins and is not required for VP16-induced complex formation. Indeed, phosphoryla tion of either CeHCF or human HCF-1 appears to be dispensable for associati on with VP16. All four CeHCF phosphorylation sites match the consensus reco gnition site for the cell-cycle kinases CDC2 and CDK2. Consistent with this similarity and with the developmental phosphorylation of CeHCF in C. elega ns embryos, CeHCF phosphorylation is cell-cycle-regulated in mammalian cell s.