Quantitative Differences between N-terminal Methionyl Nascent Globin Chains of Human and Rabbit Reticulocytes

Abstract

INCORPORATION of methionine as N-terminal residue initiates biosynthesis of a globin chain in rabbit reticulocytes^1–7. After initiation, elongation of the chain proceeds with successive additions of amino acid residues; therefore a population of actively synthesising polysomes contains nascent peptide chains of all stages of elongation⁸. The initial methionine residue is removed before completion of the chain; consequently methionine is the N-terminal residue of short nascent chains, and valine, the second amino acid incorporated, is the N-terminal residue of long nascent chains. When these chains are digested with trypsin, long nascent chains yield the same N-terminal tryptic peptides as haemoglobin, αT1 of α chain and βT1 of β chain, which have N-terminal valine and are composed of seven and eight amino acid residues, respectively^{9, 10}. The N-terminal tryptic peptides of short nascent chains are longer by one residue, namely the N-terminal methionine. Thus all but very short nascent chains contain either the methionyl or the valyl N-terminal tryptic peptide, and the ratio of yields of these peptides from a tryptic digest is a measure of the relative numbers of the two types of nascent chains in a polysomal preparation.