Characterization of ostrich (Struthio camelus) β‐microseminoprotein (MSP): Ideication of homologous sequences in EST databases and analysis of their evolution during speciation

Abstract

Abstract␤-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 and prostatic fluids, its presence was later shown in numerous secretions and its homologs were described in many vertebrate species. These studies showed that this protein had rapidly evolved, but they failed to unambiguously identify its biological role. Here, we show that a protein isolated from ostrich pituitary gland is closely related to a similar one isolated from chicken serum and that the two are structurally related to the mammalian ␤-microseminoprotein. The complete 90-amino acid sequence of the ostrich molecule was established through a combination of automated Edman degradation and matrixassisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometric procedures, including postsource decay (PSD) and ladder sequencing analyses. This study documents for the first time that ␤-microseminoprotein is present in aves. It is also the first report of a C-terminal amidated form for a member of this protein family and the first in which the disulfide linkages are established. Database searches using the herein-described amino acid sequence allowed identification of related proteins in numerous species such as cow, African clawed frog, zebrafish, and Japanese flounder. These small proteins show a strikingly high rate of amino acid substitutions, especially across phyla boundaries. Noticeably, no ␤-microseminoprotein-related gene could be found in the recently completed fruit fly genome, indicating that if such a gene exists in arthropods, it must have extensively diverged from the vertebrate ones.

Accelerated evolution of small serum proteins (SSPs)—The PSP94 family proteins in a Japanese viper

“…Even though PSP94 was first isolated as a major protein from human seminal plasma (Lilja and Abrahamsson 1988), it was later found to be present at the same level in both sexes (von der Kammer et al, 1990). PSP94 proteins have also been identified in other mammals (Fernlund et al, 1994;Xuan et al, 1999;Mäkinen et al, 1999) and in ostrich (Lazure et al, 2001). However, the amino acid sequences of these PSP94 family proteins show a limited homology.…”

Crystal Structure of Prostate Secretory Protein PSP94 Shows an Edge-to-Edge Association of two Monomers to Form a Homodimer

“…The final, refined atomic model contains 369 out of 376 amino acid residues in four polypeptide chains of PSP94. The missing region is a flexible loop (amino acids [10][11][12][13][14][15][16] in chain A. The atomic coordinates have been deposited in the Protein Data Bank (PDB) with the accession code 3IX0.…”

“…Several biological roles have been proposed since then, 16 but the exact function of PSP94 still remains elusive. The overall structure of PSP94 reported here is similar to the NMR structure reported by Ghasriani et al, but the relative orientation of the two domains is shifted by ∼ 20°.…”

“…12 PSP94 homologues have also been identified in several other mammals [13][14][15] and nonmammalian species. [16][17][18] There is a large degree of sequence diversity among these homologues, but a striking feature of PSP94 family of proteins is the presence of 10 highly conserved cysteine residues.…”

Structural analysis and characterization of new small serum proteins from the serum of a venomous snake (Gloydius blomhoffii)

“…8) Homologous proteins have been identified in several organisms. [9][10][11][12] Although the 10 cysteine residues forming five disulfide bonds were conserved, the overall amino acid similarity is only 40-50% among mammalian proteins, suggesting that they evolved at a somewhat rapid rate.…”