Amino acid composition of protein termini are biased in different manners

An exhaustive statistical analysis of the amino acid sequences at the carbo xyl (C) and amino (N) termini of proteins and of coding nucleic acid sequen ces at the 5' side of the stop codons was undertaken. At the N ends, Met an d Ala residues are over-represented at the first (+1) position whereas at p ositions 2 and 5 Thr is preferred. These peculiarities at N-termini are mos t probably related to the mechanism of initiation of translation (for Met) and to the mechanisms governing the life-span of proteins via regulation of their degradation (for Ala and Thr). We assume that the C-terminal bias fa cilitates fixation of the C ends on the protein globule by a preference for charged and Cys residues. The terminal biases, a novel feature of protein structure, have to be taken into account when molecular evolution, three-di mensional structure, initiation and termination of translation, protein fol ding and lifespan are concerned. In addition, the bias of protein termini c omposition is an important feature which should be considered in protein en gineering experiments.