The zebrafish thrombospondin 3 and 4 genes (thbs3 and thbs4): cDNA and protein structure

The cDNA sequences of the zebrafish thrombospondin 3 gene (thbs3) and thrombospondin 4 gene (thbs4) have been determined, and the encoded amino acid sequences (Thbs3 and Thbs4) have been analyzed. Thbs3 cDNA specifies a protein of 962 amino acid residues (MW 105,787), while Thbs4 cDNA specifies a protein of 949 residues (MW 103,989). Both proteins are highly acidic with Asp + Glu contents of 15.8% (Thbs3) and 16.9% (Thbs4). The acidic amino acids are clustered in the region of the Type III repeats and part of the C-terminal domain. The presence of signal peptides indicates that Thbs3 and Thbs4 function as secreted, extracellular proteins. The two polypeptides contain homologous sequences with two cysteines that are likely to form interchain disulfide bonds. Potential glycosylation sites were identified, five for Thbs3 and three for Thbs4. The polypeptides share the domain structure characteristic of the thrombospondin 3/thrombospondin 4/COMP subgroup of thrombospondin proteins. This domain structure consists of an amino-terminal thrombospondin N-terminal-like domain (TSPN domain), four Type II (EGF-like) repeats, seven Type III (calcium-binding) repeats, and a carboxyl terminal region. In aligning the Thbs3 and Thbs4 domain sequences with those of other thrombospondins, a gradient of sequence homology is observed that increases from N-terminus to C-terminus.