A high-throughput proteome of human sperm nuclei identified 403 proteins, of which more than half had not been seen in previous human sperm proteomes; the authors report the nuclei were isolated to over 99.9% purity with no acrosome, tails or mitochondria.

sp32 purified from porcine sperm binds the 55-, 53- and 49-kDa (pro)acrosin forms but not the 43-kDa intermediate or the 35-kDa mature acrosin, accelerates proacrosin autoactivation at basic pH, and redirects the maturation pathway so that the 49-kDa intermediate accumulates.

sp32 is synthesised as a 61-kDa precursor with a signal peptide whose carboxyl-terminal half becomes the mature protein, and the authors propose that its binding to proacrosin packages the zymogen into the acrosomal matrix.

The human gene was isolated as OY-TES-1 and identified as the human homologue of the proacrosin binding protein sp32 precursor previously characterised in mouse, guinea pig and pig; the Discussion adds 81.9%, 77.2% and 75.2% identity to pig, guinea pig and mouse respectively, and places the maturation cleavage between residues 273 and 274.

Expression is restricted to testis in normal tissues but occurs across a wide array of cancers, and 25 of 362 cancer patients had antibodies to the protein while none of 20 normal individuals did.

Mouse Acrbp produces wild-type ACRBP-W, processed to mature ACRBP-C, and the intron 5-retaining variant ACRBP-V5, described as absent from other mammalian ACRBPs; the two forms bind different segments of the pro-ACR C-terminus, and ACRBP-C accelerates pro-ACR autoactivation.

ACRBP-W and ACRBP-V5 colocalise with pro-ACR in the acrosomal granules of early round spermatids, whereas the mature sperm acrosome contains only ACRBP-C.

Mice lacking both ACRBP forms are severely subfertile because the acrosome is malformed; spermatids fail to form a large acrosomal granule and the acrosome fragments. Transgenic ACRBP-V5 alone rescues the malformation, assigning granule formation to that form.

Exogenously expressed ACRBP-W blocks proacrosin autoactivation in the acrosome, and the authors conclude that retaining proacrosin inactive until acrosomal exocytosis is its major function.

ACRBP-deficient mouse sperm are fewer in the oviduct after mating but retain normal head morphology and motility, and show a marked reduction in the ability to reach unfertilized oocytes; the authors attribute the subfertility to an incomplete acrosome reaction rather than to impaired migration.

The introduction states the cross-species splicing situation directly: porcine, guinea pig and human spermatogenic cells produce only the wild-type Acrbp-W mRNA, whereas mouse produces both Acrbp-W and the intron 5-retaining Acrbp-V5. This is the clearest published statement of the human negative and it post-dates the knockout paper by three years.

Antibodies against ACRBP applied to pre-capacitated boar sperm reduce capacitation and spontaneous acrosome reaction, reduce sperm-zona pellucida binding, and reduce the response to solubilized zona pellucida or SERCA inhibition, indicating a surface-accessible pool of ACRBP that primes sperm for the acrosome reaction.

The p32 tyrosine-phosphoprotein that appears in pig sperm during capacitation is sp32/ACRBP, identified by mass spectrometry and confirmed by immunoprecipitation and immunofluorescence; acrosomal labelling disappears after an ionophore-induced acrosome reaction.

ACRBP is not proteolytically processed from its 58.5-kDa precursor to the 27.5-kDa mature form in PCSK4-null mice, proacrosin then fails to autoactivate, and sperm head/acrosome malformations appear; the authors note ACRBP lacks a strong convertase consensus site, so the processing enzyme may be downstream of PCSK4.

Summarises ACRBP as an acrosomal matrix protein controlling the packaging and timed activation of proacrosin, and separates that from a second, weakly supported literature in which ACRBP/OY-TES-1/CT23 knockdown affects proliferation and migration in somatic cells.

The human ACRBP locus carries an annotated protein-coding transcript, ACRBP-204 / ENST00000536350, that is the same intron-5 read-through event as mouse ACRBP-V5: both keep the first five canonical exons with the terminal exon extended past the exon-5 donor site into intron 5, both terminate at equivalent positions, and both replace the canonical SLxQL pentapeptide with an RYRK-initiated tail.

All five reviewed ACRBP orthologs carry a cleaved signal peptide and none carries a transmembrane, intramembrane or lipidation feature, supporting a soluble acrosomal matrix assignment over acrosomal membrane.