Primary structure of the signal peptide of tropoelastin b

Elastin is a major protein of compliant connective tissue and is characterized by an amino acid composition abundant in nonpolar residues. The soluble precursor to elastin tropoelastin, is extractable in organic solvents and possesses an extensive clustering of nonpolar amino acid residues in the immediate NH2-terminal region (Foster, J. A., Shapiro, R., Voynow, P., Crombie, G., Faris, B., and Franzblau, C. (1975) Biochemistry 14, 857-864). It was, therefore, of special interest to determine whether tropoelastin requires a hydrophobic signal peptide for vectorial transport of the nascent polypeptide. The possibility that the initial tropoelastin translation product possesses a short signal peptide was examined in a cell-free translation system. Total RNA, isolated from aortae of 1-day-old chicks, was translated in an mRNA-dependent reticulocyte lysate translation assay. The translation products were then immunoprecipitated and subjected to automated radiosequencing. Comparison of the NH2-terminal sequence of tropoelastin b synthesized in the cell-free system versus that synthesized in organ culture demonstrated the presence of a signal peptide 24 amino acid residues in length. The signal peptide sequence is as follows: Met-Arg-Gln-Ala-Ala-Ala-Pro-Leu-Leu-Pro-Gly-Val-Leu-Leu-Leu-Phe-Ser-Ile-Leu-Pro -Ala-Ser-Gln-Gln. The preponderance of hydrophobic amino acid residues as well as the polar residues adjacent to the initiator methionine and the carboxyl termini found in the signal peptide is similar to that reported for other secreted proteins.