The human 64-kDa polyadenylylation factor contains a ribonucleoprotein-type RNA binding domain and unusual auxiliary motifs

Cleavage stimulation factor is one of the multiple factors required for 3′-end cleavage of mammalian pre-mRNAs. We have shown previously that this factor is composed of three subunits with estimated molecular masses of 77, 64, and 50 kDa and that the 64-kDa subunit can be UV-crosslinked to RNA in a polyadenylylation signal (AAUAAA)-dependent manner. We have now isolated cDNAs encoding the 64-kDa subunit of human cleavage stimulation factor. The 64-kDa subunit contains a ribonucleoprotein-type RNA binding domain in the N-terminal region and a repeat structure in the C-terminal region in which a pentapeptide sequence (consensus MEARA/G) is repeated 12 times and the formation of a long α-helix stabilized by salt bridges is predicted. An ≈270-amino acid segment surrounding this repeat structure is highly enriched in proline and glycine residues (≈20% for each). When cloned 64-kDa subunit was expressed in Escherichia coli, an N-terminal fragment containing the RNA binding domain bound to RNAs in a polyadenylylation-signal-independent manner, suggesting that the RNA binding domain is directly involved in the binding of the 64-kDa subunit to pre-mRNAs.