Amino-terminal processing of proteins: hemoglobin South Florida, a variant with retention of initiator methionine and N alpha-acetylation.

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Abstract
The hemoglobin variant South Flordia has been shown by protein sequencing and fast-atom-bombardment mass spectroscopy to have a substitution of methionine for the NH2-terminal valine of the .beta.-globin chain. In addition, there was complete retention of the initiator methionine on the mutant polypeptide. Approximately 20% of the protein was acetylated at the NH2 terminus of the .beta. chain. A search of a comprehensive data bank of protein and gene sequences revealed 84 unreleated vertebrate proteins that have not undergone cleavage of leader sequences. A highly nonrandom distribution of residues at the NH2 termini of these proteins predicts removal of the initiator methionine as well as NH2-terminal acetylation. Proteins that undergo removal commonly have serine, alanine, glycine, or valine, as the NH2-terminal residues. The first three residues favor N.alpha.-acetylation. Proteins that retain the initiator methionine commonly have a charged residue or methionine at the second position. Information on Hb South Florida and other hemoglobins coupled with this survey of primary sequence provides insights into the NH2-terminal processing of proteins.