Identification of a Novel 81-kDa Component of the Xenopus Origin Recognition Complex

The Xenopus origin recognition complex is essential for chromosomal DNA replication in cell-free extracts. We have immunopurified the Xenopus origin recognition complex with anti-Xorc2 antibodies and analyzed its composition and properties. Xorc2 (p63) is specifically associated with Xorc1 (p115) and up to four additional polypeptides (p81, p78, p45, and p40). The cDNA encoding p81 is highly homologous to various expressed sequence tags from humans and mice encoding a protein of previously unknown function. Immunodepletion of p81 fromXenopus egg extracts, which also results in the removal of Xorc2, completely abolishes chromosomal DNA replication. Thus, p81 appears to play a crucial role at S phase in higher eukaryotes.