Hunnicutt G R, Koppel D E, Myles D G
Center for Biological Research, The Population Council, 1230 York Avenue, New York, New York 10021, USA.
Dev Biol. 1997 Nov 1;191(1):146-59. doi: 10.1006/dbio.1997.8700.
Fertilin is a heterodimeric (subunits alpha and beta) sperm plasma membrane protein. Both subunits belong to the ADAM protein family of surface proteins that contain a disintegrin and a metalloprotease domain. Fertilin functions in sperm-egg fusion by binding the sperm to the egg plasma membrane via a binding site in the disintegrin domain of fertilin beta. On testicular sperm of guinea pig, fertilin is distributed on the plasma membrane over the entire sperm head, but is found only on the posterior head once sperm have passed through the epididymis. This redistribution of fertilin to the posterior head can be partially mimicked in vitro if testicular sperm are briefly treated with trypsin. In this study we used immunofluorescence and digital image analysis to analyze how fertilin becomes restricted to the posterior head. We found that fertilin became restricted to the posterior head by migration of anterior head fertilin molecules into the posterior head domain. Comparison of immunofluorescence patterns and immunoblots of fertilin from seven regions of the epididymis showed a temporal correlation between the beginning of fertilin's migration to the posterior head and the proteolytic processing of the full-length fertilin beta precursor (the 85-kDa pro-beta form) to a 75-kDa intermediate, pro-beta*. Completion of the migration coincided with the further cleavage of pro-beta* to the 25- to 28-kDa mature form. Our data suggest that the cleavage of fertilin pro-beta to pro-beta* may initiate fertilin's migration into the posterior head domain and, after localization to that membrane domain, pro-beta* is cleaved to mature beta. We also report evidence that a common mechanism may be used to change the localization pattern of other sperm surface molecules. Other surface proteins were shown to become localized to either the posterior or the anterior head membrane domains on sperm at the same time fertilin became localized to the posterior head. These restrictions of surface protein localizations were also shown to immediately precede the development of the sperm's ability to swim and undergo the acrosome reaction, and thus redistribution of surface proteins may be necessary before sperm become functional.
受精素是一种异源二聚体（α和β亚基）精子质膜蛋白。两个亚基都属于ADAM表面蛋白家族，该家族包含一个解整合素和一个金属蛋白酶结构域。受精素通过受精素β解整合素结构域中的结合位点将精子与卵质膜结合，从而在精卵融合中发挥作用。在豚鼠的睾丸精子上，受精素分布在整个精子头部的质膜上，但精子通过附睾后，仅在后头部发现。如果用胰蛋白酶短暂处理睾丸精子，受精素向精子后头部的这种重新分布在体外可以部分模拟。在本研究中，我们使用免疫荧光和数字图像分析来分析受精素如何局限于后头部。我们发现，前头部的受精素分子迁移到后头部区域，使得受精素局限于后头部。对来自附睾七个区域的受精素的免疫荧光模式和免疫印迹进行比较，结果显示受精素迁移到后头部开始与全长受精素β前体（85 kDa的前β形式）蛋白水解加工为75 kDa的中间体前β之间存在时间相关性。迁移完成与前β进一步切割为25至28 kDa的成熟形式同时发生。我们的数据表明，受精素前β切割为前β可能启动受精素向后头部区域的迁移，并且在定位到该膜区域后，前β被切割为成熟的β。我们还报告了证据表明，可能使用一种共同机制来改变其他精子表面分子的定位模式。在受精素定位于后头部的同时，其他表面蛋白也定位于精子的后头部或前头部膜区域。这些表面蛋白定位的限制也显示在精子获得游泳能力和发生顶体反应之前立即出现，因此表面蛋白的重新分布可能是精子具备功能之前所必需的。
