A CPSF-73 homologue is required for cell cycle progression but not cell growth and interacts with a protein having features of CPSF-100

Formation of the mature 3′ ends of the vast majority of cellular mRNAs occurs through cleavage and polyadenylation and requires a cleavage and polyadenylation specificity factor (CPSF) containing, among other proteins, CPSF-73 and CPSF-100. These two proteins belong to a superfamily of zinc-dependent β-lactamase fold proteins with catalytic specificity for a wide range of substrates including nucleic acids. CPSF-73 contains a zinc-binding histidine motif involved in catalysis in other members of the β-lactamase superfamily, whereas CPSF-100 has substitutions within the histidine motif and thus is unlikely to be catalytic-ally active. Here we describe two previously unknown human proteins, designated RC-68 and RC-74, which are related to CPSF-73 and CPSF-100 and which form a complex in HeLa and mouse cells. RC-68 contains the intact histidine motif, and hence it might be a functional counterpart of CPSF-73, whereas RC-74 lacks this motif, thus resembling CPSF-100. In HeLa cells RC-68 is present in both the cytoplasm and the nucleus whereas RC-74 is exclusively nuclear. RC-74 does not interact with CPSF-73, and neither RC-68 nor RC-74 is found in a complex with CPSF-160, indicating that these two proteins form a separate entity independent of the CPSF complex and are likely involved in a pre-mRNA processing event other than cleavage and polyadenylation of the vast majority of cellular pre-mRNAs. RNA interference-mediated depletion of RC-68 arrests HeLa cells early in G ₁ phase, but surprisingly the arrested cells continue growing and reach the size typical of G₂ cells. RC-68 is highly conserved from plants to humans and may function in conjunction with RC-74 in the 3′ end processing of a distinct subset of cellular pre-mRNAs encoding proteins required for G¹ progression and entry into S phase.