Amino acid composition of protein termini are biased in different manners

An exhaustive statistical analysis of the amino acid sequencesat the carboxyl (C) and amino (N) termini of proteins and ofcoding nucleic acid sequences at the 5' side of the stop codonswas undertaken. At the N ends, Met and Ala residues are over-representedat the first (+1) position whereas at positions 2 and 5 Thris preferred. These peculiarities at N-termini are most probablyrelated to the mechanism of initiation of translation (for Met)and to the mechanisms governing the life-span of proteins viaregulation of their degradation (for Ala and Thr). We assumethat the C-terminal bias facilitates fixation of the C endson the protein globule by a preference for charged and Cys residues.The terminal biases, a novel feature of protein structure, haveto be taken into account when molecular evolution, three-dimensionalstructure, initiation and termination of translation, proteinfolding and life-span are concerned. In addition, the bias ofprotein termini composition is an important feature which shouldbe considered in protein engineering experiments.