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 2 proteins differ only in this respect. On treatment with cyanogen bromide these proteins gave 3 classes of peptides with mol wt of 30,000, 10,000 and ≤ 5,000. A combination of gel filtration, electrofocusing, and ion exchange established that these peptides were indistinguishable, with exception of those of 10,000 mol wt. 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.