Binding of mannose‐6‐phosphate and heparin by boar seminal plasma PSP‐II, a member of the spermadhesin protein family

Abstract

PSP‐I/PSP‐II, a heterodimer of glycosylated spermadhesins, is the major component of boar seminal plasma. Similarly to other spermadhesins, the PSP‐II subunit is a lectin which displays heparin‐ and zona pellucida glycoprotein‐binding activities. We have investigated the ligand binding capabilities of the heterodimer and the isolated subunits using several polysaccharides, glycoproteins, and phospholipids. PSP‐II binds the sulfated polysaccharides heparin and fucoidan in a dose‐dependent and seemingly‐specific manner. In addition, PSP‐II binds oligosaccharides containing exposed mannose‐6‐phosphate monoester groups and the binding is selectively inhibited by mannose‐6‐phosphate and glucose‐6‐phosphate. Inhibition experiments indicate that binding of PSP‐II to sulfated polysaccharides and mannose‐6‐phosphate‐containing oligosaccharides involves distinct but possibly overlapping binding sites. Heterodimer formation with PSP‐I abolishes both the heparin and the mannose‐6‐phosphate binding capabilities, suggesting that the corresponding sites may be located at the dimer interface. Using the crystal structure of PSP‐I/PSP‐II heterodimer as a template, we have explored possible binding sites which satisfy the observed binding characteristics. In the proposed models, PSP‐II Arg⁴³ appears to play a pivotal role in both heparin‐ and mannose‐6‐phosphate‐complexation as well as in heterodimer formation.