We have defined the nucleotide sequence of a protein-binding domain within Ul RNA that specifically recognizes and binds both to a Ul small nuclear ribonucleoprotein component (the 70K protein) and to the previously defined RNA-binding domain of the 70K protein. We have investigated direct interactions between purified Ul RNA and 70K protein by reconstitution in vitro. Thirty-one nucleotides of Ul RNA, corresponding to stem-loop I, were required for this interaction. Nucleotides at the 5' end of Ul RNA that are involved in base pairing with the 5' splice site of pre-mRNA were not required for binding. In contrast to other reports, these findings demonstrate that a specific domain of Ul RNA can bind directly to the 70K protein independently of any other snRNP-associated proteins.The 52-kilodalton Ul RNA-associated protein (the 70K protein) is a component of the Ul small nuclear ribonucleoprotein (snRNP) complex. Several snRNPs have been implicated in the removal of intervening sequences during precursor mRNA splicing (see references 11, 25, and 41 for reviews). The Ul snRNP, in particular, has been shown to interact with the 5' splice site, at least in part by base pairing of this site with the 5'-terminal 10 nucleotides (nt) of Ul RNA (1,5,18,29,45). The human Ul snRNP is composed of 165 nt of Ul RNA and two classes of proteins: Ul-specific proteins (70K, A, and C) and the Sm complex (consisting of six U snRNP-common proteins) (9, 22). Although these proteins probably play important structural roles in snRNPs and at least some are required for the interaction of Ul snRNP with 5' splice sites (29), none have been assigned specific functions.Previous studies of the RNA-binding properties of the Ul snRNP-specific proteins have involved indirect methods including immunoprecipitation of nuclease-treated cell extracts (34) and microinjection of RNA into Xenopus oocytes (12). These studies suggested that the 70K protein, as well as the A protein and perhaps the C protein, associates with the 5' half of Ul RNA, in particular, stem-loop I. In addition, weak requirements were implicated for regions of stemloops II, III, and IV (12,34 purified human Ul RNA and 70K protein, we have investigated the direct interaction of these two components of the Ul snRNP. Using various binding methods, we maintained the specificity of the Ul RNA-70K reconstitution through progressive deletion of both components. We previously defined the RNA-binding domain of the 70K protein required for this interaction (35). In this study, we report that both the 70K protein and the RNA-binding domain alone are directly recognized by 31 nt of Ul RNA that make up the 5'-most stem-loop structure. The A, C, and Sm proteins were not required for this recognition and binding. Further deletion of the 31-nt structure resulted in reduced efficiency of reconstitution, thus defining the minimal RNA domain needed for recognition of the 70K protein.
MATERIALS AND METHODS