NH2-terminal sequence of the chick proalpha1(I) chain synthesized in the reticulocyte lysate system. Evidence for a transient hydrophobic leader sequence.

Translation of chick procollagen mRNA in a reticulocyte lysate system yields a larger proalpha1(I) chain than is observed in vivo. The NH2-terminal sequence of this putative precursor, determined by automated radiosequencing, is Met-Phe-Ser-Phe-Val-X-Ser-Arg-Leu-Leu-Leu-Leu-Ile-Ala-Ala-X-X-Leu-Leu. This sequence closely resembles the transient hydrophobic leader (signal) sequences observed on most secreted proteins. When synthesized in the presence of microsomal membranes from dog pancreas, which contain signal peptidase activity, proalpha chains with the electrophoretic mobility of underhydroxylated procollagen polypeptides synthesized in vivo are produced. Estimates of the molecular weight of the NH2-terminal extension of the precursor suggest that the leader sequence may be longer than those commonly found in precursors of secreted proteins.