MOLECULAR EVOLUTION OF OLFACTOMEDIN

Olfactomedin is a secreted polymeric glycoprotein of unknown function, originally discovered at the mucociliary surface of the amphibian olf actory neuroepithelium and subsequently found throughout the mammalian brain. As a first step toward elucidating the function of olfactomedi n, its phylogenetic history was examined to identify conserved structu ral motifs. Such conserved motifs may have functional significance and provide targets for future mutagenesis studies aimed at establishing the function of this protein. Previous studies revealed 33% amino acid sequence identity between rat and frog olfactomedins in their carboxy l terminal segments. Further analysis, however, reveals mole extensive homologies throughout the molecule. Despite significant sequence dive rgence, cysteines essential for homopolymer formation such as the CXC motif near the amino terminus are conserved, as is the characteristic glycosylation pattern, suggesting that these posttranslational modific ations are essential for function. Furthermore, evolutionary analysis of a region of 53 amino acids of fish, frog, rat, mouse, and human olf actomedins indicates that an ancestral olfactomedin gene arose before the evolution of terrestrial vertebrates and evolved independently in teleost, amphibian, and mammalian lineages. Indeed, a distant olfactom edin homolog was identified in Caenorhabditis elegans. Although the am ino acid sequence of this invertebrate protein is longer and highly di vergent compared with its vertebrate homologs, the protein from C. ele gans shows remarkable similarities in terms of conserved motifs and po sttranslational modification sites. Six universally conserved motifs w ere identified, and five of these are clustered in the carboxyl termin al half of the protein. Sequence comparisons indicate that evolution o f the N-terminal half of the molecule involved extensive insertions an d deletions; the C-terminal segment evolved mostly through point mutat ions, at least during vertebrate evolution. The widespread occurrence of olfactomedin among vertebrates and invertebrates underscores the no tion that this protein has a function of universal importance. Further more, extensive modification of its N-terminal half and the acquisitio n of a C-terminal SDEL endoplasmic-reticulum-targeting sequence may ha ve enabled olfactomedin to adopt new functions in the mammalian centra l nervous system.