The structure of rat proalbumin.

The structure of rat proalbumin, a liver precursor to rat serum albumin, has been determined to consist of the hexapeptide Arg-Gly-Val-Phe-Arg-Arg attached to the NH2 terminus of the polypeptide chain of rat serum albumin. Edman degradation of a proalbumin preparation for 14 rounds gave the major sequence Arg-Gly-Val-Phe-Arg-Arg-Glu-Ala-His-Lys-Ser-Glu-Ile-Ala. A comparison of cyanogen bromide fragments suggests that these two proteins differ only in this respect. On treatment with cyanogen bromide, these proteins gave three classes of peptides with molecular weights of 30,000, 10,000, and smaller than or equal to 5,000. A combination of gel filtration, electrofocusing, and ion exchange established that these peptides were indistinguishable, with exception of those of 10,000 molecular weight. By amino acid and sequence analyses this fraction from rat serum albumin was found to be the NH2-terminal fragment. Radiochemical amino acid and sequence analyses show that the NH2-terminal hexapeptide is the major fragment released from proalbumin by limited tryptic hydrolysis. The protein that remains cannot be distinguished from rat serum albumin.