A determination of the absolute amounts of high-mobility-group proteins 1 and 2 (HMG1 and HMG2) in rat tissues demonstrated that amounts of HMG2 were low in non-proliferating tissues, somewhat higher in proliferating and lymphoid tissues, but were extremely elevated in the testis. This increase was due to a germ-cell-specific form of HMG2 with increased mobility relative to somatic HMG2 on acid/urea/polyacrylamide-gel electrophoresis. To determine if the findings in the rat were a general feature of spermatogenesis, testis (germinal), spleen (lymphoid), and liver (non-proliferating) tissues from various vertebrate species were examined for their relative amounts of HMG1 and HMG2, and for HMG2 heterogeneity. Bull, chimpanzee, cynomologus monkey, dog, gopher, guinea pig, hamster, mouse, opossum, rabbit, rat, rhesus monkey, squirrel and toad (Xenopus) tissues were analysed. Nearly all species showed relatively high contents of HMG2 in testis tissue, whereas HMG1 contents were similar in all species and tissues. Ten of thirteen species showed a rapidly migrating HMG2 subtype in testis tissue, separable by acid/urea/polyacrylamide-gel electrophoresis. Xenopus, which lacks HMG2 in somatic tissues, showed an HMG2-like protein in testis tissue. Although the rapidly migrating HMG2 subtype in species other than rat was not testis-specific, it was always enriched in the testis. This study indicates that increased amounts of HMG2 and the enrichment of a rapidly migrating HMG2 subtype are general features of spermatogenic cells.

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