Two Functional Forms of ACRBP/sp32 Are Produced by Pre-mRNA Alternative Splicing in the Mouse1

Abstract

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.

Biogenesis of sperm acrosome is regulated by pre-mRNA alternative splicing ofAcrbpin the mouse

“…The frequencies of the midpiece bending in type 1/type 2 and type 3 Acrbp −/− sperm were modestly and severely reduced, respectively. The abnormal bending motion in Acrbp −/− sperm may be due to the deformed structure of the acrosome because ACRBP is localized exclusively in the acrosome (18). We could not examine the type 4 sperm, because of the entwinement of the midpiece around the nucleus (Fig.…”

“…ACRBP also has been identified as a member of the cancer/testis antigen family; ACRBP is normally expressed exclusively in the testis, but is also expressed in a wide range of different tumor types, including bladder, breast, liver, and lung carcinomas (15)(16)(17). Mammalian ACRBP is initially synthesized as a ∼60-kDa precursor protein (ACRBP-W) in spermatogenic cells, and the 32-kDa mature ACRBP (ACRBP-C) is posttranslationally produced by removal of the N-terminal half of the precursor ACRBP-W during spermatogenesis and/or epididymal maturation of sperm (14,18). In the mouse, two forms of Acrbp mRNA, wild-type Acrbp-W and variant Acrbp-V5 mRNAs, are produced by pre-mRNA alternative splicing of Acrbp (18).…”

“…Mammalian ACRBP is initially synthesized as a ∼60-kDa precursor protein (ACRBP-W) in spermatogenic cells, and the 32-kDa mature ACRBP (ACRBP-C) is posttranslationally produced by removal of the N-terminal half of the precursor ACRBP-W during spermatogenesis and/or epididymal maturation of sperm (14,18). In the mouse, two forms of Acrbp mRNA, wild-type Acrbp-W and variant Acrbp-V5 mRNAs, are produced by pre-mRNA alternative splicing of Acrbp (18). Similarly to other mammalian ACRBP-W, mouse ACRBP-W starts to be synthesized in pachytene spermatocytes and immediately processed into ACRBP-C (Fig.…”

“…The intron 5-retaining splice variant mRNA produces a predominant form of ACRBP, ACRBP-V5, that is also present in pachytene spermatocytes and round spermatids, but is absent in elongating spermatids (18). Functional analysis in vitro reveals that ACRBP-V5 and ACRBP-C possess a different domain capable of binding each of two segments in the C-terminal region of proACR (18). Moreover, autoactivation of proACR is remarkably accelerated by the presence of Significance Mammalian sperm possess a Golgi-derived exocytotic organelle, the acrosome, located on the apical region of the head.…”

“…ACRBP-C (14,18). Thus, we postulated that, at least in the mouse, ACRBP-V5 and ACRBP-C may differentially function in the transport/packaging of proACR into the acrosomal granule during spermiogenesis and in the promotion of ACR release from the acrosome during acrosomal exocytosis, respectively.…”