Evolution of Protein Physical Structures in Insect Chemosensory Systems

Abstract

Insect chemosensory protein (CSP) structures are built of six-seven α-helices, four cysteines making two adjacent disulfide bridges forming a multifunction prism for transport of lipid chains and small insecticide chemicals. Moth pheromone binding proteins (PBPs) have bowl-like globular structures made of six α-helices; six cysteines forming three interlocked disulfide bridges. Niemann-Pick type C2 proteins mediating chemical communication in ants display β-barrel structures, similar to mammalian lipocalins and odor binding proteins (OBPs). How do all these structures relate to each other from an evolutionary standpoint? What was the folding of the ancestral “chemosensory” molecule? A close overview of “chemosensory” protein structures described in insects suggests that addition of cysteine residues has played a key role in the evolution of function in the vast functional repertoire of binding proteins. In addition, motif insertion, motif inversion, duplication of amino acid pairs and specific residue substitution in typical locations of the protein structure might have been essential to lead to new protein structures and new functions. Importantly, addition of key residues such as glycine near conserved cysteine residues might lead to insertion or deletion of secondary structural elements, depending on the protein family. The chapter presented here describes the multi-level aspects of mutations that govern evolution and function in the vast repertoire of binding protein families. I try to understand evolution of these protein structures and functions using both RNA and peptide mutations recently discovered in the Bombyx system.