Molecular Biology of the Zona Pellucida

ZP2和ZP3共定位于内质网和由多泡聚集体(MVA)组成的1-5微米高尔基体后结构中，但免疫共沉淀试验不能检测到物理相互作用。此外，ZP2在缺乏ZP3或ZP3发生突变以阻止整合到透明带基质中的情况下，在发育中的卵母细胞中正常运输，补充了早期关于ZP3在ZP2基因缺失小鼠中独立分泌的研究。N-糖基化与正确的蛋白质折叠和分泌蛋白质的细胞内运输有关。虽然ZP3含有5个N-糖链，但在MVA中存在缺乏N-糖基化位点的绿色荧光蛋白标记的ZP3，并被整合到转基因小鼠的小带基质中。因此，ZP2的分泌似乎不受缺乏N-糖链的ZP3的影响。综上所述，这些观察表明，ZP2和ZP3在组装到细胞外透明带之前独立地通过卵母细胞进行运输。 分类群特异性精子-卵子识别的分子基础仍有待确定。小鼠精子非常混杂，并与人类卵子结合，但人类精子不会与小鼠卵子结合。小鼠透明带由ZP1、ZP2和ZP3组成，ZP1、ZP2和ZP3在大鼠和人中都是保守的。最近观察到人类透明带含有第四种蛋白质，这增加了四种透明带蛋白质的存在将支持人类精子结合的可能性。利用质谱仪，在大鼠卵透明带中检测到4种与人ZP1、ZP2、ZP3和ZP4/ZPB大小相似、氨基酸同源性为62-70%的蛋白质。然而，尽管小鼠和大鼠的精子与每种啮齿类动物的卵子结合，但人类精子两者都不结合。此外，与正常小鼠卵子相比，缺乏杂合/复合N-糖链或缺乏核心20-O-糖链的突变小鼠卵子并不能更好地支持人类精子结合。这些数据表明，四种带状蛋白的存在不足以支持人类精子与啮齿动物卵子的结合，而且其他决定因素肯定对哺乳动物之间的分类群特异性受精负责。

ZP2 and ZP3 co-localize in the endoplasmic reticulum and in 1-5 um post-Golgi structures comprised of multivesicular aggregates (MVA), but a co-immunoprecipitation assay does not detect physical interactions. In addition, ZP2 trafficks normally in growing oocytes in the absence of ZP3 or if ZP3 has been mutated to prevent incorporation into the zona matrix, complementing earlier studies indicating the independence of ZP3 secretion in Zp2 null mice. N-glycosylation has been implicated for correct protein folding and intracellular trafficking of secreted proteins. Although ZP3 contain five N-glycans, EGFP-tagged ZP3 lacking N-glycosylation sites is present in MVA and is incorporated into the zona matrix of transgenic mice. Thus, ZP2 secretion is seemingly unaffected by ZP3 lacking N-glycans. Taken together, these observations indicate that ZP2 and ZP3 traffick independently through the oocyte prior to assembly into the extracellular zona pellucida. The molecular basis of taxon-specific sperm-egg recognition remains to be determined. Mouse sperm are quite promiscuous and bind to human eggs, but human spermatozoa will not bind to mouse eggs. The mouse zona pellucida is composed of ZP1, ZP2 and ZP3 which are conserved in rat and human. The recent observation that human zonae pellucidae contain a fourth protein raised the possibility that the presence of four zona proteins will support human sperm binding. Using mass spectrometry, four proteins that are similar in size and share 62-70% amino acid identity with human ZP1, ZP2, ZP3 and ZP4/ZPB were detected in rat zonae pellucidae. However, although mouse and rat spermatozoa bind to eggs from each rodent, human sperm bind to neither. In addition, mutant mouse eggs lacking hybrid/complex N-glycans or deficient in Core 2 O-glycans were no more able to support human sperm binding than normal mouse eggs. These data suggest that the presence of four zona proteins are not sufficient to support human sperm binding to rodent eggs and that additional determinants must be responsible for taxon-specific fertilization among mammals.