Homology of the precursor of pulmonary surfactant-associated protein SP-B with prosaposin and sulfated glycoprotein 1.

The precursor of pulmonary surfactant-associated protein, SP-B, is composed of an NH2-terminal domain of 30 residues (a-type domain) and three tandem repeats of about 90 residues (b-type domain); biophysically active mature SP-B corresponds to the second b-type repeat. Consensus sequences constructed for the b-type repeats were used to search the data base for homologous sequences, and the search has revealed that prosaposin and sulfated glycoprotein 1 show a remarkable homology with these repeats. The domain organizations of the latter proteins, however, differ from that of SP-B precursor inasmuch as they contain four tandem copies of the b-type domain and a-type domains are present both in the NH2-terminal and COOH-terminal parts of the proteins. The implications of the homology of saposins and SP-B for their structure and function are discussed.