A multicomponent complex is required for the AAUAAA-dependent cross-linking of a 64-kilodalton protein to polyadenylation substrates.

Abstract

A 64-kilodalton (kDa) polypeptide that is cross-linked by UV light specifically to polyadenylation substrate RNAs containing a functional AAUAAA element has been identified previously. Fractionated HeLa nuclear components that can be combined to regenerate efficient and accurate polyadenylation in vitro have now been screened for the presence of the 64-kDa protein. None of the individual components contained an activity which could generate the 64-kDa species upon UV cross-linking in the presence of substrate RNA. It was necessary to mix two components, cleavage stimulation factor and specificity factor, to reconstitute 64-kDa protein-RNA cross-linking. The addition of cleavage factors to this mixture very efficiently reconstituted the AAUAAAspecific 64-kDa protein-RNA interaction. The 64-kDa protein, therefore, is present in highly purified, reconstituted polyadenylation reactions. However, it is necessary to form a multicomponent complex to efficiently cross-link the protein to a substrate RNA.The maturation of the 3' ends of most RNA polymerase II transcripts involves an endonucleolytic cleavage event followed by the polymerization of 150 to 200 adenylate residues onto the newly formed terminus (reviewed in references 2 and 16). This cleavage-polyadenylation event is mediated through two cis-acting sequence elements: a highly conserved hexanucleotide, AAUAAA, located 5 to 30 bases upstream of the cleavage site (8,12,22,27,35,37), and a less conserved U-or GU-rich stretch located downstream of the cleavage site (6,7,10,17,19,20,28,30,31). The selective recognition of alternative polyadenylation sites may play a role in the posttranscriptional regulation of gene expression (reviewed in reference 14).Progress in understanding the mechanism of polyadenylation has been facilitated greatly by the development of accurate and faithful in vitro systems (15,24). The recognition of the polyadenylation signal occurs through the formation of a large, multicomponent complex. Native gel and density gradient analyses have shown that a 40 to 50S complex forms in an AAUAAA-dependent manner on substrate RNAs (13,25,32,33,39,40). Immunological and biochemical studies suggest that small nuclear RNAs play some role in this complex (1,11,13,(24)(25)(26). Label transfer experiments with UV light have identified proteins of 64 to 68 and 155 kilodaltons (kDa) which interact with RNAs that contain a polyadenylation signal (23, 38). Additionally, biochemical fractionation of the nuclear extract has identified several independent components which are required for reconstitution of in vitro processing (3, 4, 9, 18, 21, 34, 34a).We have now assayed partially purified polyadenylation components (34, 34a) for the presence of the 64-kDa protein by UV cross-linking. None of the individual fractions contained an activity that could be cross-linked to RNA to generate the 64-kDa species. However, a mixture of at least * Corresponding author. A flow chart of the biochemical purification steps performed is shown in Fig. 1. Details of the purificati...

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

“…6). These results agree with those obtained with extensively purified CstF (22) and support the view that the 64-kDa subunit does not recognize the AAUAAA polyadenylylation signal.…”

“…These findings are consistent with previous results obtained with extensively purified CstF (19,22), which, coupled with functional studies (13,14), led to the suggestion that CstF does not directly recognize AAUAAA (e.g., ref. 18 (20,21), and our preliminary experiments with purified CstF or the recombinant 64-kDa subunit are consistent with these earlier findings.…”

“…By immunoprecipitation with a monoclonal antibody (mAb) against the 64-kDa subunit, this polypeptide was shown (18) to be identical to a protein of 64-68 kDa that had been detected (20,21) in crude nuclear extracts by UV-crosslinking to pre-mRNAs in an AAUAAA sequence-dependent manner. Since both CPSF and CstF are required for this specific UV-crosslinking (19,22) and for the formation of a stable complex on pre-mRNAs (14,23), it has been suggested that CstF interacts with both the pre-mRNA and CPSF to stabilize the interaction between the pre-mRNA and CPSF. To understand the molecular mechanisms of CstF function, we cloned cDNAs encoding the 64-kDa subunit and determined its primary structureA We also expressed the cloned cDNA in Escherichia coli and examined the RNA binding of the purified protein.…”

Localization of RNAPII and 3′ end formation factor CstF subunits onC. elegansgenes and operons

“…[33][34][35]56,57 However, very little is known about the in vivo assembly of the complex. Since both CstF subunits peak at 3 0 ends of genes similar to RNA-PII, we wanted to determine whether the levels of CstF64 and CstF50 are similar relative to total RNA-PII.…”

GRSF-1: a poly(A)+mRNA binding protein which interacts with a conserved G-rich element

“…Processing of the 3' end of pre-mRNAs is also mediated by RNA -protein interactions. The interplay between two cis-acting elements (AAUAAA and a U-rich downstream element) and trans-acting factors (CPSF and CstF) may determine the location and efficiency of 3' end cleavage and polyadenylation (9)(10)(11).…”