Nucleic acid binding characteristics of group A/B hnRNP proteins

hnRNP proteins are primarily defined by their specific sedimentational, reconstitutional, and extraction properties and are presumed to be RNA binding. However, it is not clear whether all these proteins have RNA binding capabilities. Recently, using two monoclonal antibodies, fA12 and AC88, we reported that the abundance of a subclass of the highly basic A/B hnRNP proteins was specifically down regulated during terminal differentiation of human and murine cells in vitro. In this report we have examined the nucleic acid binding characteristics of this subclass and other members of the A/B hnRNP proteins in vitro. All members of class A/B hnRNP proteins appear to have similar but not identical nucleic acid binding characteristics. However, the subclass of proteins recognized by AC88 and fA12 exhibit stronger binding affinities and are shown to be highly selective in their binding to RNA vs DNA in vitro. These proteins also preferentially bind poly(U) RNA, suggesting that in vivo they may bind effectively to uridine rich motifs critical in pre-mRNA processing.