FUNCTION OF CONSERVED DOMAINS OF HNRNP A1 AND OTHER HNRNP A B PROTEINS/

hnRNP A1 is a pre-mRNA binding protein that antagonizes the alternativ e splicing activity of splicing factors SF2/ASF or SC35, causing activ ation of distal 5' splice sites. The structural requirements for hnRNP A1 function were determined by mutagenesis of recombinant human hnRNP A1. Two conserved Phe residues in the RNP-1 submotif of each of two R NA recognition motifs appear to be involved in specific RNA-protein in teractions and are essential for modulating alternative splicing. Thes e residues are not required for general pre-mRNA binding or RNA anneal ing activity. The C-terminal Gly-rich domain is necessary for alternat ive splicing activity, for stable RNA binding and for optimal RNA anne aling activity. hnRNP A1(B), which is an alternatively spliced isoform of hnRNP A1 with a longer Gly-rich domain, binds more strongly to pre -mRNA but has only limited alternative splicing activity. In contrast, hnRNP A2 and B1, which have 68% amino acid identity with hnRNP A1, bi nd more weakly to pre-mRNA and have stronger splice site switching act ivities than hnRNP A1. We propose that specific combinations of antago nistic hnRNP A/B and SR proteins are involved in regulating alternativ e splicing of distinct subsets of cellular pre-mRNAs.