Protein 4.1 is a multifunctional polypeptide that links transmembrane proteins with the underlying spectrin/actin cytoskeleton. Recent studies have shown that protein 4.1 is also present in the nucleus, localized in domains enriched in splicing factors. Here we further analyze the relationship between protein 4. 1 and components of the splicing machinery. Using HeLa nuclear extracts capable of supporting the splicing of pre-mRNAs in vitro, we show that anti-4.1 antibodies specifically immunoprecipitate pre-mRNA and splicing intermediates. Immunodepletion of protein 4.1 from HeLa nuclear extracts results in inhibition of their splicing activity, as assayed with two different pre-mRNA substrates. Coprecipitation of protein 4.1 from HeLa nuclear extracts with proteins involved in the processing of pre-mRNA further suggests an association between nuclear protein 4.1 and components of the splicing apparatus. The molecular cloning of a 4.1 cDNA encoding the isoform designated 4.1E has allowed us to show that this protein is targeted to the nucleus, that it associates with the splicing factor U2AF35, and that its overexpression induces the redistribution of the splicing factor SC35. Based on our combined biochemical and localization results, we propose that 4.1 proteins are part of nuclear structures to which splicing factors functionally associate, most likely for storage purposes.

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