Molecular characterization of a karyophilic, histone-binding protein: cDNA cloning, amino acid sequence and expression of nuclear protein N1/N2 of Xenopus laevis.

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Abstract
In the amphibian oocyte, most of the non-chromatin-bound histones are not free but form complexes with specific karyophilic proteins, the most prominent being nucleoplasmin and ‘protein N1/N2’. Using antibodies against polypeptide N1 and N2 (Mr approximately 105,000 and approximately 110,000) we have isolated, from a Xenopus laevis ovary lambda gt11 expression library, several full length cDNA clones encoding one of the two closely related polypeptides N1 and N2 (these could not be distinguished by hybridization techniques). The amino acid sequence deduced from one of these clones (N1/N2, lambda 106.2) defines a polypeptide of mol. wt 64,774. The remarkably high difference between the value of Mr approximately 110,000 estimated from SDS-PAGE mobility and the true mol. wt has been found for (i) the cell protein, (ii) the polypeptide synthesized in vitro by transcription and translation and (iii) the fusion protein with beta-galactosidase expressed in Escherichia coli, indicating that the protein runs anomalously on SDS-PAGE. The amino and carboxy termini of the purified protein N1/N2 have been confirmed by direct amino acid sequencing of CNBr fragments. The amino acid sequence displays two glutamic acid-rich domains, which are probably involved in the interaction with the histones, and a putative nuclear targeting signal with high homology to that of the SV40 large T-antigen which is located near the carboxy terminus.(ABSTRACT TRUNCATED AT 250 WORDS)