Studies of the strand-annealing activity of mammalian hnRNP complex protein A1.

A1 is a major core protein of the mammalian hnRNP complex, and as a purified protein of approximately 34 kDa, A1 is a strong single-stranded nucleic acid binding protein. Several lines of evidence suggest that the protein is organized in discrete domains consisting of an N-terminal segment of approximately 22 kDa and a C-terminal segment of approximately 12 kDa. Each of these domains as a purified fragment is capable of binding to both ssDNA and RNA. We report here that A1 and its C-terminal domain fragment are capable of potent strand-annealing activity for base-pair complementary single-stranded polynucleotides of both RNA and DNA. This effect is not stimulated by ATP. Compared with A1 and the C-terminal fragment, the N-terminal domain fragment has negligible annealing activity. These results indicate that A1 has biochemical activity consistent with a strand-annealing role in relevant reactions, such as pre-mRNA splicing.