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 proteolytic cleavage
are particularly mobile. Their amino acid compositions reveal a common and
unusual abundance of alanine, accompanied by high levels 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. Moreover, the known 3D structures of an uncleaved serpin reactive
centre and a fusion peptide are strikingly similar. Such sequences possess many
small residues and are found in several signal peptides and in PrP, a protein
associated with spongiform encephalopathies and resembling virus envelope
proteins. These properties may be related to the infection mechanisms of these
diseases.