SR proteins and hnRNP H regulate the splicing of the HIV‐1 tev‐specific exon 6D

A naturally arising point mutation in the env gene of HIV‐1 activates the aberrant inclusion of the cryptic exon 6D into most viral messages, leading to inefficient viral replication. We set out to understand how a single nucleotide substitution could cause such a dramatic change in splicing. We have determined that the exon 6D mutation promotes binding of the SR protein SC35 to the exon. Mutant exon 6D sequences function as a splicing enhancer when inserted into an enhancer‐dependent splicing construct. hnRNP H family proteins bind to the enhancer as well; their binding is dependent on the sequence GGGA located just downstream of the point mutation and depletion– reconstitution studies show that hnRNP H is essential for enhancer activity. A polypurine sequence located further downstream in exon 6D binds SR proteins but acts as an exonic splicing silencer. hnRNP H is required for interaction of U1 snRNP with the enhancer, independent of the point mutation. We propose that SC35 binding to the point mutation region may convert the hnRNP H–U1 snRNP complex into a splicing enhancer.