A sperm motility-controlling protein adapted to the urodele mode of internal fertilization and its diversity among amphibian species

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Sperm motility-initiating substance is a nonglycosilated protein that acts specifically in the mode of internal fertilization of the urodele amphibian, Cynops pyrrhogaster. In the present study, a cDNA encoding SMIS protein was identified. It coded a sequence of 151 amino acids that contained a cysteine knot domain. The expression of the SMIS gene was oviduct-specific and independent of the stimuli of ovulation. The active site of the SMIS was detected in a hydrophilic loop of the cysteine knot domain using synthetic peptides having partial amino acid sequences of the SMIS. Immunoblotting and southernblotting revealed that proteins homologous to SMIS were widely conserved among anuran and urodele amphibians but those homologues showed different isoelectric points ranging from 4 to 7. These results suggest that the molecular feature of the SMIS may be changed in adaptation to the diversified fertilization modes among amphibian species.