Characterization of ostrich (Struthio camelus) beta-microseminoprotein (MSP): Identification of homologous sequences in EST databases and analysis oftheir evolution during speciation

beta -Microseminoprotein, alternatively called prostatic secretory protein of 94 amino acids, is a hydrophilic, unglycosylated, small protein rich in conserved half-cystine residues. Originally found in human seminal plasma a nd prostatic fluids, its presence was later shown in numerous secretions an d its homologs were described in many vertebrate species. These studies sho wed that this protein had rapidly evolved, but they failed to unambiguously identify its biological role. Here, we show that a protein isolated from o strich pituitary gland is closely related to a similar one isolated from ch icken ser-um and that the two are structurally related to the mammalian bet a -microseminoprotein. The complete 90-amino acid sequence of the ostrich m olecule was established through a combination of automated Edman degradatio n and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF ) mass spectrometric procedures, including postsource decay (PSD) and ladde r sequencing analyses. This study documents for the first time that beta -m icroseminoprotein is present in aves. It is also the first report of a C-te rminal amidated form for a member of this protein family and the first in w hich the disulfide linkages are established, Database searches using the he rein-de scribed amino acid sequence allowed identification of related prote ins in numerous species such as cow, African clawed frog, zebrafish, and Ja panese flounder. These small proteins show a strikingly high rate of amino acid substitutions, especially across phyla boundaries. Noticeably, no beta -microseminoprotein-related gene could be found in the recently completed fruit fly genome. indicating that if such a gene exists in arthropods, it m ust have extensively diverged from the vertebrate ones.