Evolution of olfactomedin. Structural constraints and conservation of primary sequence motifs

Olfactomedin is a glycosylated extracellular matrix protein originally identified at the mucociliary surface of the amphibian olfactory neuroepithelium and subsequently localized throughout the mammalian central nervous system. Although olfactomedin homologues have been identified in fish, frog, rat, mouse and human, its function is still unknown. As a first step toward elucidating the function of olfactomedin, sequences of teleost, amphibian and human homologues were compared to identify invariant, and hence, potential functionally important motifs. Previous studies revealed 33% amino acid sequence identity between rat and frog olfactomedin in their carboxyl terminal segments. Further analysis, however, reveals more extensive homologies throughout the molecule. Despite significant sequence divergence, cysteines essential for homo-polymer formation, such as the CXC motif near the amino terminus, are conserved as is the characteristic glycosylation pattern, suggesting that these posttranslational modifications are essential for function. Furthermore, alignment of a region of 53 amino acids of fish, frog, rat and human olfactomedin reveals seven invariant residues including a negatively charged cluster of aspartic and glutamic acid residues. Molecular evolutionary genetic analysis reveals an accelerated rate of nucleotide substitutions in the mammalian lineage. The evolutionary rate at the protein level, however, is constant, indicating that evolution of olfactomedin is constrained by structural limitations. Whereas considerable evolutionary divergence is evident between fish, frog and mammalian olfactomedins, olfactomedins of rat and human show 98% amino acid sequence identity. It appears that an ancestral olfactomedin gene arose before the evolution of terrestrial vertebrates and evolved independently in teleost, amphibian and mammalian lineages. The apparent evolutionary pressure toward conservation of primary structure supports the notion that olfactomedin has an important function in the mammalian nervous system.