Human 100-kDa homologous DNA-pairing protein is the splicing factor PSF and promotes DNA strand invasion

Proteins promoting homologous pairing could be involved in various fundamen tal biological processes. Previously we detected two mammalian nuclear prot eins of 100 and 75 kDa able to promote homologous DNA pairing. Here we repo rt isolation and characterisation of the human (h) 100-kDa DNA-pairing prot ein, hPOMp100, from HeLa nuclei. The peptide sequences of hPOMp100 revealed identity to the human splicing factor PSF and a DNA-binding subunit of p10 0/p52 heterodimer of unknown function. Bacterially expressed PSF promotes D NA pairing identical to that of hPOMp100. hPOMp100/PSF binds not only RNA b ut also both single-stranded (ss) and double-stranded (ds) DNA and facilita tes the renaturation of complementary ssDNAs. More important, the protein p romotes the incorporation of a ss oligonucleotide into a homologous superhe lical dsDNA, D-loop formation. A D-loop is the first heteroduplex DNA inter mediate generated between recombining DNA molecules, Moreover, this reactio n could be implicated in re-establishing stalled replication forks. Consist ent with this hypothesis, DNA-pairing activity of hPOMp100/PSF is associate d with cellular proliferation. Significantly, phosphorylation of hPOMp100/P SF by protein kinase C inhibits its binding to RNA but stimulates its bindi ng to DNA and D-loop formation and may represent a regulatory mechanism to direct this multifunctional protein to DNA metabolic pathways.