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One candidate for the sperm factor was “oscillin,” a protein identified in hamster
sperm. Oscillin was isolated by serial chromatographic purification, and when injected
into mouse eggs, it was able to induce a series of Ca2+ oscillations (Parrington et al.
1996 ). It is located in the equatorial segment region of the sperm head, which is the
expected location of a signaling molecule that is to trigger the fertilization Ca2+ signal
after gamete fusion. Its amino acid sequence showed 53 % identity to a bacterial glu-
cosamine-6-phosphate deaminase (Shevchenko et al. 1998 ). However, recombinant
oscillin, while exhibiting deaminase activity, was unable to induce Ca2+ oscillations
after being injected into mammalian eggs (Wolosker et al. 1998 ). This indicated that
a sperm extract factor other than oscillin was the much sought-after molecule that
caused activation.
In vitro assays demonstrated that mammalian sperm extracts had high PLC
enzyme activity, even at relatively low Ca2+ concentrations typical of mammalian
eggs awaiting fertilization (Rice et al. 2000 ). This suggested that the active factor in
sperm might be a PLC isoform. In fact, mammalian spermatozoa express several
known PLC isoforms (Fukami 2002 ). However, recombinant forms of these pro-
teins failed to trigger Ca2+ oscillations when injected into eggs (Parrington et al.
2002 ) or did so only at very high, nonphysiological concentrations (Mehlmann et al.
2001 ). In addition, chromatographic fractionation of sperm extracts showed that
none of the known PLC isoforms were present in the fraction that possessed the
Fig. 1.3 Different hypotheses describing alternative mechanisms by which the fertilizing sperm
stimulates the release of Ca2+ from the endoplasmic reticulum (ER endoplasmic reticulum, IP 3
inositol 1,4,5-trisphosphate, PLC phospholipase C, SF sperm factor; from Nomikos et al. 2011 ,
with permission)
Z. Machaty et al.