The K10/10.1 protein is encoded by a cluster of interferon regulatory factor (IRF) homologues in the Kaposi's sarcoma-associated herpesvirus (KSHV, human herpesvirus 8, HHV-8) genome. In the present study, we showed that an anti-K10 antibody reacted with a 110-kDa protein encoded by the K10/10.1 gene of KSHV in KSHV-infected primary effusion lymphoma (PEL) cell lines. Expression of K10/10.1 protein was induced by phorbol ester in KSHV-infected cells. A reporter gene assay demonstrated that K10/10.1 protein did not influence promoter activity of human interferon genes, regardless of its homology to human IRFs. Poly(A)-binding protein (PABP) was identified as a partner of K10/10.1 protein. Immunoprecipitation revealed that K10/10.1 protein interacted with PABP specifically in PEL cell lines. IFA revealed co-localization of K10/10.1 protein and PABP in the nucleus of KSHV-infected cells. These data suggest that K10/10.1 protein may affect the translational status or stability of mRNA in host cells.