Localization of the transglutaminase cross-linking site in SVSIII, a novel glycoprotein secreted from mouse seminal vesicle

The nucleotide sequence of MpSv-1, a novel androgen-regulated gene exclusively expressed in mouse seminal vesicle, was analyzed to establish a 5'-flanking region of 2123 bp, three exons of 95, 765, and 330 bp, and two introns of 222 and 811 bp. The transcription unit is organized with the first exon encoding a signal peptide, and the second a secreted protein, whereas the third encompasses a T-non-translated nucleotide that shares common features of rapid evolving substrates of transglutaminase gene family. The protein sequence deduced from this gene contains 265 amino acid residues in which the central part, residues 116-145, is a region composed of five short tandem repeats, consisting of four amino acid residues, QXK(S/T), where X is an aliphatic amino acid residue. Among the mouse seminal vesicle secretory proteins that could be resolved by SDS-PAGE into seven major components, SVS I-VII, the antiserum against residues 77-109 of the MpSv-1-translated protein only reacted with SVS III. Matrix-assisted laser desorption/ionization-time of flight mass spectral analysis from a trypsin digest of SVS III supported this protein as derived from MpSv-1. SVS III was immunolocalized to the epithelium of both the primary and secondary folds of the seminal vesicle and the copulatory plug. All of mouse SVS I-IH were proven to be substrates of transglutaminase and could be cross-linked readily after the enzyme reaction. The transglutaminase cross-linking site of SVS III was identified to be the tandem repeats of QXK(S/T) in the central part of this protein molecule.