Single stranded DNA binding proteins derive from hnRNP proteins by proteolysis in mammalian cells

As we have previously demonstrated, mammalian single stranded DNA binding proteins (ssDBP) and heterogeneous nuclear RNA binding proteins (hnRNP proteins) are antigenically and structurally related. In this paper we show that ssDBP are specific proteolytic products of hnRNP core proteins. Proteolysis can be observed in crude extract, both total and nuclear and is not inhibited by the most commonly used protease inhibitors. Such phenomenon can be observed in HeLa cells, human fibroblasts and calf thymus extracts. A trypsin-like protease that cleaves purified hnRNP proteins to give ssDBP of Mr = 24-28 Kd can be purified from HeLa cells. A precursor-product relationship can be established between hnRNP core proteins type A and an ssDBP of 24 Kd (UP1).