COMMON PREVALENCE OF ALANINE AND GLYCINE IN MOBILE REACTIVE CENTER LOOPS OF SERPINS AND VIRAL FUSION PEPTIDES - DO PRIONS POSSESS A FUSION PEPTIDE

Serpin reactive centre loops and fusion peptides released by proteolyt ic cleavage are particularly mobile. Their amino acid compositions rev eal a common and unusual abundance of alanine, accompanied by high lev els of glycine. These two small residues, which are not simultaneously abundant in stable helices (standard or transmembrane), probably play an important role in mobility. Threonine and valine (also relatively small amino acids) are also abundant in these two kinds of peptides. M oreover, the known 3D structures of an uncleaved serpin reactive centr e and a fusion peptide are strikingly similar. Such sequences possess many small residues and are found in several signal peptides and in Pr P, a protein associated with spongiform encephalopathies and resemblin g virus envelope proteins. These properties may be related to the infe ction mechanisms of these diseases.