ACRBP/sp32 is a binding protein specific for the precursor (pro-ACR) and intermediate forms of sperm serine protease ACR. In this study, we examined the expression pattern, localization, and possible role of mouse ACRBP in spermatogenic cells and epididymal sperm. Unlike other mammalian ACRBPs, two forms of Acrbp mRNA—wild-type Acrbp-W and variant Acrbp-V5 mRNAs—were generated by alternative splicing of Acrbp in the mouse. ACRBP-W was synthesized in pachytene spermatocytes and haploid spermatids and immediately processed into a mature protein, ACRBP-C, by removal of the N-terminal half. The intron 5-retaining splice variant mRNA produced a predominant form of ACRBP, ACRBP-V5, that was present in pachytene spermatocytes and round spermatids, but was absent in elongating spermatids. ACRBP-W and ACRBP-V5 were both colocalized with pro-ACR in the acrosomal granules of early round spermatids, whereas the sperm acrosome contained only ACRBP-C. Glutathione S-transferase pull-down assays revealed that ACRBP-V5 and ACRBP-C possess a different domain capable of binding each of two segments in the C-terminal region of pro-ACR. Moreover, autoactivation of pro-ACR was remarkably accelerated by the presence of ACRBP-C. These results suggest that ACRBP-V5 and ACRBP-C may function in the transport/packaging of pro-ACR into acrosomal granules during spermiogenesis and in the promotion of ACR release from the acrosome during acrosomal exocytosis, respectively.
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Vol. 88 • No. 4