Chick a-tectorin: Molecular cloning and expression during embryogenesis.

The avian and mammalian tectorial membranes both contain two non- collagenous glycoproteins, a and -tectorin. To determine whether variations in the primary sequences of the chick and mouse a-tectorins account for differences in subunit composition and matrix structure of the tectorial membranes in these two species, cDNAs spanning the entire open reading frame of chick a-tectorin were cloned and the derived amino acid sequence was compared with that of mouse a-tectorin. Chick a-tectorin shares 73% amino acid sequence identity with mouse a-tectorin and, like mouse a-tectorin, is composed of three distinct modules: an N-terminal region similar to the G1 domain of entactin, a central region that shares identity with zonadhesin and contains three full and two partial von Willebrand factor type D repeats, and a D-terminal region containing a zona pellucida domain. The central region of chick a-tectorin contains fewer potential N-glycosylation sites than that of mouse a-tectorin and is cleaved at two additional sites. Differences in the glycosylation and proteolytic processing of chick and mouse a-tectorin may therefore account for the variation observed in the composition and structure of the collagenase- insensitive matrices of the avian and mammalian tectorial membranes. In situ hybridisation and Northern blot analysis of chick inner ear tissue indicate that the spatial and temporal patterns of a and -tectorin mRNA expression in the developing chick inner ear are different, suggesting the two tectorins may each form homomeric filaments.