Characterization of HeLa nuclear factors which interact with a conditionally processed rat fibronectin pre-mRNA.

It is demonstrated that several HeLa nuclear proteins interact in a magnesium-dependent fashion with a conditionally processed pre-mRNA derived from an alternatively spliced region of the rat fibronectin gene. HnRNP C proteins crosslink under both permissive and non-permissive conditions (high and low magnesium, respectively), whereas hnRNP I/PTB is observed only under the latter. Thus hnRNP I/PTB represents a potential inhibitor of splicing for this intron. A protein that was observed to crosslink under permissive conditions has been identified as splicing factor U2AF65, which thus represents a candidate positive factor in the splicing of this pre-mRNA. We interpret these results with a working model for the conditional selection of the 3' splice site of alternative exon EIIIB.