The primary structure of transition protein 4 (TP4) from boar late spermatid nuclei was determined by automated Edman degradation of S-pyridylethylated protein and of peptides generated by cleavage with Staphylococcus aureus V8 protease, lysyl endopeptidase and CNBr. Boar TP4 is a basic protein consisting of a highly basic amino-terminal half (residues 1-73) and a less basic carboxy-terminal half (residues 74-1 38). The latter half includes a highly hydrophobic segment, a four-times tandemly repeated sequence, N(G)QNKR(K)X, and a carboxy-terminal segment containing Trpl26. Ultraviolet absorption and CD spectra of TP4 -rat-liver-nucleosome-core-DNA (double-stranded DNA) complexes suggest a TP4-induced local melting of DNA. Although at 1 mM NaCl TP4 brought about a slight stabilization of the DNA against thermal melting, a destabilization of the DNA was observed at 50 mM NaCl. From the results of quenching of tryptophan (Trp126) fluorescence of TP4 upon its binding to double-stranded and single-stranded boar liver nucleosome-core DNA at 50 mM NaCl, the apparent association constants for the binding of TP4 to double-stranded and single-stranded DNA were calculated to be 7.3X10' M-' and 4.1 X 10' M -', respectively. These results suggest that TP4, having different domain structures from TP1-3 and a higher affinity for double-stranded DNA, induces a local destabilization of DNA probably through the stacking of Trp126 with nucleic acid bases.Keywords: transition protein ; amino acid sequence; DNA-protein interaction; late spermatid nuclei.The mechanism of transformation of nucleosome-type chromatin into a nucleoprotamine fiber is rather poorly understood. A direct transition from a nucleosome-type chromatin organization into a nucleoprotamine fiber operates in most species with the exception of mammals [I, 21. In mammals, nucleosomal histones are transiently replaced by small basic proteins called transition proteins (TPI -4) and, finally, by protamines [3-61. At the same time, transformation of the nucleosomal-type chromatin into a smooth chromatin fiber, initiation of chromatin condensation and cessation of transcription occur [7-121. The amino acid sequences of TPI and TP2, and the nucleotide sequences of their genes are known [ 13 -161, and attempts have been made to understand how they are transcriptionally and translationally regulated [17, 181. It has been postulated that rat TP1 is a DNA melting protein, mediated through the intercalation of its tyrosine residue between the nucleic acid bases [19], that it destabilizes the compact nucleosome core particles 1201, and that rat TP2, with two possible zinc fingers in the amino- terminal region, is a DNA stabilizing protein [21, 221. Recently, we have developed methods for isolating intact boar TP1, TP3 and TP4 123, 241, and have reported that boar TP3 has 27% sequence similarity to boar TPI [25]. In this paper, we describe the amino acid sequence of boar TP4 and the nature of interaction of TP4 with nucleosome-core DNA in vitro.
MATERIALS AND METHODSMaterials. sta...