PHOSPHORYLATION OF HUMAN REPLICATION PROTEIN-A BY THE DNA-DEPENDENT PROTEIN-KINASE IS INVOLVED IN THE MODULATION OF DNA-REPLICATION

The single-stranded DNA-binding protein, Replication Protein A (RPA), is a heterotrimeric complex with subunits of 70, 32 and 14 kDa involve d in DNA metabolism, RPA may be a target for cellular regulation; the 32 kDa subunit (RPA32) is phosphorylated by several cellular kinases i ncluding the DNA-dependent protein kinase (DNA-PK), We have purified a mutant hRPA complex lacking amino acids 1-33 of RPA32 (rhRPA . 32 Del ta 1-33). This mutant bound ssDNA and supported DNA replication; howev er, rhRPA . 32 Delta 1-33 was not phosphorylated under replication con ditions or directly by DNA-PK, Proteolytic mapping revealed that all t he sites phosphorylated by DNA-PK are contained on residues 1-33 of RP A32. When wild-type RPA was treated with DNA-PK and the mixture added to SV40 replication assays, DNA replication was supported, In contrast , when rhRPA . 32 Delta 1-33 was treated with DNA-PK, DNA replication was strongly inhibited. Because untreated rhRPA . 32 Delta 1-33 is ful ly functional, this suggests that the N-terminus of RPA is needed to o vercome inhibitory effects of DNA-PK on other components of the DNA re plication system, Thus, phosphorylation of RPA may modulate DNA replic ation indirectly, through interactions with other proteins whose activ ity is modulated by phosphorylation.