Stage-specific expression of proprotein form of hyaluronan binding protein 1 (HABP1) during spermatogenesis in rat

The presence of the 34-kDa hyaluronan binding protein 1 (HABP1) on sperm surface and its role in fertilization is already established (Ranganathan et al., 1994: Mol Reprod Dev 38:69-76). In the present communication, we examined the expression of HABP1 in adult rat testis during spermatogenesis. Interestingly, using anti-rHABP1 antibody, we detected a protein of 55 kDa which was present only in testis, but not in other somatic tissues like spleen and liver. However, even in testis, only one transcript of HABP1 mRNA of 1.63 kb was observed. In addition, we confirm that this testis-specific 55 kDa protein was immunologically identical with proprotein form of HABP1 using antibody raised against a decapeptide present in the proprotein region of HABP1. Comparative immunohistochemistry of testis, spleen, and liver tissues using both the antibodies supported the observation that the proprotein form of HABP1 is present only in testis. Higher mRNA expression of HABP1 in testis as compared to that of liver and spleen could be speculated from the RT-PCR product. Finally, detailed study of the immunohistochemical staining of the seminiferous tubules revealed the expression of the HABP1 proprotein in specific stages of germ cells, like pachytene spermatocytes and round spermatids, but not in elongated ones, suggesting a possible role of HABP1 proprotein in spermatogenic differentiation.