RNA ANNEALING ACTIVITIES IN HELA NUCLEI

RNA-RNA base pairing plays a critical role in the interactions between pre-mRNAs and trans-acting factors during the processing of pre-mRNAs (hnRNAs) into mRNAs, and it is likely that specific factors are requi red to promote the annealing of RNAs. To identify particular nuclear c omponents that have such activity, we fractionated HeLa nucleoplasm an d assayed for activity which promoted the hybridization of a pre-mRNA with an antisense RNA probe complementary to 60 nucleotides (nt) encom passing the 3' splice site. At least nine major RNA annealing activiti es were identified and, surprisingly, eight of these copurified partia lly or to homogeneity with known hnRNP proteins. The activities of thr ee of these proteins, hnRNP A1, C1 and U, were confirmed using purifie d recombinant proteins. Moreover, we found that the RNA binding domain alone of hnRNP C1/C2 had significant activity, indicating that this R NA annealing may result, at least partly, from chaperone activity: a d irect modulation of RNA conformation by hnRNP proteins. The finding th at hnRNP proteins have strong RNA annealing activity indicates that th ey can profoundly affect the interactions of pre-mRNAs with trans-acti ng factors and suggests this to be an important function of hnRNP prot eins in the processing of pre-mRNAs.