A sperm motility inhibitor isolated from porcine seminal plasma is identical to porcine β-microseminoprotein (MSP). Circular dichroism (CD) and nuclear magnetic resonance (NMR) data showed that the native and recombinant porcine MSPs exhibit very similar structure. The five disulfide pairings on porcine MSP were unambiguously assigned based on NMR data and further confirmed using structural calculations. Surprisingly, our derived pairings differ from those recently reported for ostrich MSP based on matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) analysis. Furthermore, the secondary structure was determined to comprise one four-stranded and two double-stranded antiparallel β-sheets. As we know, this is the first detailed secondary structure reported among several types of MSPs.
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