Most proteins are visualized on the polyacrylamide gel after electrophoretic separation by staining with dyes. However, some proteins that are often heavily glycosylated are invisible with the conventional methods. In our laboratory, we have focused on gametic cells-specific glycoproteins to gain an insight into biological significance of glycan chains at fertilization, and eventually encountered two kinds of invisible glycoproteins. [1] One is a group of GPI-anchored sperm glycoproteins with an unusually high carbohydrate content (80-90% by weight). Flagellasialin from sea urchin sperm contains a short polypeptide (7.2 kDa) and six O-glycans with an α2,9-linked poly-N-acetylneuraminic acid chain, and is involved in the regulation of intracellular Ca2+, [Ca2+]i, and sperm motility. Regulation of the [Ca2+]i is an important physiological event in animal sperm. We then asked if flagellasialin-like molecules also occur in other animal sperm, and finally found the presence of such glycoproteins in pig and chicken sperm. They are commonly localized in membrane microdomains (or lipid rafts), and share the same potential function with flagellasialin; i.e., when a single pig sperm is treated with a specific antibody to the pig sperm molecule (CD52), the transient increase of [Ca2+]i is observed. Interestingly, despite a common feature for overall structure, structures of the mature protein and glycans are extensively different among species. [2] The other group is a glycan-rich egg glycoprotein localized in secretory vesicles, or cortical alveoli (CA), and collectively named hyosophorin. In CA, hyosophorin exists as a high-molecular-weight form (H-hyosophorin) consisting of tandem repetitions of heavily glycosylated short peptides, and depolymerized into the least repeating glycopeptide (L-hyosophorin) at the perivitelline space during CA exocytosis at fertilization. Since L-hyosophorin, but not H-hyosophorin, has the cell proliferation activity, L-hyosophorin is recognized to be an active molecule during embryogenesis. Interestingly, both peptide sequence and glycan structures of L-hyosophorin are extremely different between species, although a short peptide with a high carbohydrate content (80-90%) is a common structural feature. We think it challenging to elucidate structure-function relationships of these two types of glycan-rich glycoproteins in sperm and eggs.

Invisible glycoproteins with unusually high carbohydrate content in animal gametic cells

254th National Meeting (2017)

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