Chick alpha-tectorin: molecular cloning and expression during embryogenesis

The avian and mammalian tectorial membranes both contain two non-collagenous glycoproteins, alpha and beta-tectorin. To determine whether variations in the primary sequences of the chick and mouse alpha-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 alpha-tectorin were cloned and the derived amino acid sequence was compared with that of mouse alpha-tectorin. Chick alpha-tectorin shares 73% amino acid sequence identity with mouse alpha-tectorin and, like mouse alpha-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 C-terminal region containing a zona pellucida domain. The central region of chick alpha-tectorin contains fewer potential N-glycosylation sites than that of mouse alpha-tectorin and is cleaved at two additional sites. Differences in the glycosylation and proteolytic processing of chick and mouse alpha-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 alpha and beta-tectorin mRNA expression in the developing chick inner ear are different, suggesting the two tectorins may each form homomeric filaments.