The mouse tectorins: modular matrix proteins of the inner ear homologous to components of the sperm-egg adhesion system.

The cDNA and derived amino acid sequences for the two major non- collagenous proteins of the mouse tectorial membrane, a- and -tectorin, are presented. The cDNA for a-tectorin predicts a protein of 239,034 Da with 33 potential N-glycosylation sites, and that of -tectorin a smaller protein of 36,074 Da with 4 consensus N-glycosylation sites. Southern and Northern blot analysis indicate a- and -tectorin are single copy genes only expressed in the inner ear, and in situ hybridization shows they are expressed by cells both in and surrounding the mechanosensory epithelia. Both sequences terminate with a hydrophobic COOH terminus preceded by a potential endoproteinase cleavage site suggesting the tectorins are synthesized as glycosylphosphatidylinositol-linked, membrane bound precursors, targeted to the apical surface of the inner ear epithelia by the lipid and proteolytically released into the extracellular compartment. The mouse - tectorin sequence contains a single zona pellucida domain, whereas a- tectorin is composed of three distinct modules: an NH2-terminal region similar to part of the entactin G1 domain, a large central segment with three full and two partial von Willebrand factor type D repeats, and a carboxyl- terminal region which, like -tectorin, contains a single zona pellucida domain. The central, high molecular mass region of a-tectorin containing the yon Willebrand factor type D repeats has homology with zonadhesin, a sperm membrane protein that binds to the zona pellucida. These results indicate the two major non-collagenous proteins of the tectorial membrane are similar to components of the sperm-egg adhesion system, and, as such may interact in the same manner.