Mouse sperm protein sp56 is a component of the acrosomal matrix

Previously, we identified the guinea pig sperm acrosomal matrix glycoprotei n AM67 and demonstrated that it is most closely related to mouse sperm sp56 , initially reported to be a cell-surface protein. On the contrary, our stu dies demonstrated that sp56 is an intra-acrosomal component, Based upon the homology between guinea pig AM67 and mouse sp56, we hypothesized that sp56 was part of the acrosomal matrix, a structure that had yet to be demonstra ted to exist in mouse sperm, In this paper, we show that sp56 first appeare d in late meiotic cells and accumulated during spermiogenesis, the haploid stage of spermatogenic cell development. Using affinity-purified anti-pepti de antisera, we determined that the molecular weight of sp56 in cauda epidi dymal sperm approximated that of guinea pig AM67 (similar to 67 000 M-r) an d that sp56 was present in a high molecular weight, disulfide-linked comple x. The forms of sp56 in pachytene spermatocytes and spermatids had higher m olecular weights than was found for the sperm form; the size differences we re apparently due to alterations in carbohydrate side chains. The sp56 comp lex could not be solubilized by the nonionic detergent Triton X-100 but rem ained associated with the dorsal surface of the mouse sperm head, demonstra ting that sp56 is a component of the mouse sperm acrosomal matrix.