Characterization of high mobility group protein levels during spermatogenesis in the rat

The distribution, quantitation, and synthesis of high mobility group (HMG) proteins during spermatogenesis in the rat have been determined. HMG1, -2, -14, and -17 were isolated from rat testes by Bio-Rex 70 chromatography combined with preparative gel electrophoresis. Amino acid analysis revealed that each rat testis HMG protein was similar to its calf thymus analogue. Tryptic peptide maps of somatic and testis HMG2 showed no differences and, therefore, failed to detect an HMG2 variant. Testis levels of HMG proteins, relative to DNA content, were equivalent to other tissues for HMG1 (13 micrograms/mg of DNA), HMG14 (3 micrograms/mg of DNA), and HMG17 (5 micrograms/mg of DNA). The testis was distinguished in that it contained a substantially higher level of HMG2 than any other rat tissue (32 micrograms/mg of DNA). HMG protein levels were determined from purified or enriched populations of testis cells representing the major stages of spermatogenesis; spermatogonia and early primary spermatocytes, pachytene spermatocytes, early spermatids, and late spermatids; and testicular somatic cells. High levels of HMG2 in the testis were due to pachytene spermatocytes and early spermatids (56 +/- 4 and 47 +/- 6 micrograms/mg of DNA, respectively). Mixtures of spermatogonia and early primary spermatocytes showed lower levels of HMG2 (12 +/- 3 micrograms/mg of DNA) similar to proliferating somatic tissues, whereas late spermatids had no detectable HMG proteins. The somatic cells of the testis, including isolated populations of Sertoli and Leydig cells, showed very low levels of HMG2 (2 micrograms/mg of DNA), similar to those in nonproliferating somatic tissues. HMG proteins were synthesized in spermatogonia and primary spermatocytes, but not in spermatids. Rat testis HMG2 exhibited two bands on acid-urea gels. A "slow" form comigrated with somatic cell HMG2, while the other "fast" band migrated ahead of the somatic form and appeared to be testis-specific. The "fast" form of HMG2 accounted for the large increase of HMG2 levels in rat testes. These results show that the very high level of HMG2 in testis is not associated with proliferative activity as previously hypothesized.