The testis-specific high-mobility-group protein, a phosphorylation-dependent DNA-packaging factor of elongating and condensing spermatids.
Mammalian spermiogenesis is characterized by a striking restructuring of the spermatid chromatin caused by the replacement of nucleohistones with transition proteins and their subsequent replacement with nucleoprotamines. The onset of nuclear elongation and chromatin condensation in spermatids is accompanied by a general decrease in the transcriptional activity of the DNA. A recently identified testis-specific high-mobility-group (tsHMG) protein, similar to the human mitochondrial transcription factor I and to the linker-associated protein delta of Tetrahymena thermophila micronuclei, is thought to play a structural role in this process. We confirm by immunoblot analysis of fractionated germ cells that the presence of tsHMG is restricted to transcriptionally quiescent elongating and condensing spermatids. Purified recombinant tsHMG protein displays preferential binding to supercoiled plasmid DNA, which reversibly protects the DNA against the DNA-relaxing activity of eukaryotic topoisomerase I and also impairs the transcriptional activity of this template when assayed in vitro. The tsHMG protein can also introduce negative supercoils into a relaxed plasmid substrate in a topoisomerase I-dependent manner. We also show that the tsHMG protein is the substrate of a Ca2+-phospholipid-dependent protein kinase (protein kinase C) present in testis extracts of adult mice and demonstrate that phosphorylation by protein kinase C is required for both the DNA-binding and the topoisomerase I-dependent supercoiling activities of tsHMG. Our results support the hypothesis that the spermatid tsHMG protein is a topological factor (transition protein) that can modulate the activity of topoisomerase I. This activity could contribute to the important transition in chromatin structure which leads to the decrease in DNA metabolism observed at the early stages of spermatid elongation.
Alami-Ouahabi
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N
Department of Biochemistry, Faculty of Medicine, University of Sherbrooke, Quebec, Canada.
Veilleux
S
S
Meistrich
M L
ML
Boissonneault
G
G
D000818:Animals; D004278:DNA, Superhelical / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism; D005786:Gene Expression Regulation; D006609:High Mobility Group Proteins / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism*; D006801:Humans; D008297:Male; D051379:Mice; D024101:Mitochondrial Proteins; D009687:Nuclear Proteins; D010766:Phosphorylation; D010957:Plasmids / Q000378:metabolism; D015800:Protozoan Proteins / Q000378:metabolism; D011994:Recombinant Proteins / Q000302:isolation & purification / Q000378:metabolism; D013087:Spermatids / Q000502:physiology*; D013090:Spermatocytes / Q000378:metabolism; D013091:Spermatogenesis; D013093:Spermatogonia / Q000378:metabolism; D016808:Tetrahymena thermophila / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism; D014158:Transcription, Genetic
D004278:DNA, Superhelical; D004268:DNA-Binding Proteins; D006609:High Mobility Group Proteins; D024101:Mitochondrial Proteins; D009687:Nuclear Proteins; D015800:Protozoan Proteins; D011994:Recombinant Proteins; C499631:TFAM protein, human; C081930:Tfam protein, mouse; D014157:Transcription Factors; C422562:mitochondrial transcription factor A