背景與目的: 不孕是重要的公眾健康議題，造成不孕的因素其中一半與男性相關。其中造成男性不孕的一項重要病因是畸型精子症。在先前的研究中，我們以cDNA微晶片比較造精功能正常與異常的睪丸組織中各別基因的表現量。其中發現一個與不孕相關Rab GTPase激活蛋白，名為TBC1D21蛋白。TBC1D21特異的表現於減數分裂後期精細胞中。而於Tbc1d21基因剔除小鼠模式中發現嚴重精子尾部的中節區域損壞。進一步我們以蛋白質質體學方法鑑定出與TBC1D21結合蛋白。於本研究中我們聚焦於其中一個TBC1D21結合蛋白RAB5C。研究方法與結果: RAB5C蛋白表現於小鼠睪丸組織切片中的精子細胞的頂體與尾部。於成熟精子中，RAB5C蛋白濃縮於頭部頂體與散落於尾部中節區域。其中有趣的是精子經過頂體反應後，於頂體的RAB5C蛋白量明顯下降消失。最後我們利用共同免疫染色法確認RAB5C與TBC1D21共同散落於成熟精子尾部中節區域。結論:基於以上結果，我們發現TBC1D21交互作用蛋白RAB5C參與於精子頭部頂體與尾部中節的形成。

Background and purpose: Infertility is a critical public health issue, and half of its causes are related to male factors. Teratozoospermia is one of major cause of male infertility. In our previous studies, we identified the TBC1 domain family member 21 (TBC1D21) gene, a Rab GTPase-activating protein (GAP), as a sterility-related gene in cDNA microarray compared to normal and spermatogenic testicular tissues. TBC1D21 is specifically expressed in post-meiotic male germ cells. Furthermore, the loss of Tbc1d21 allele in mice results in severe defects in the midpiece of the sperm’s tail. Additionally, TBC1D21 interactors were selected using proteomic assays. Herein, we focused on RAB5C, which is a TBC1D21 interactor. Methods and Results: RAB5C was localized in the acrosomal region and elongating tail in spermatids of murine testicular sections. In a mature sperm cell, RAB5C signals were concentrated in the acrosome and were scattered around the midpiece. Interestingly, RAB5C protein levels decreased in the acrosome after triggering acrosomal reaction. Finally, we confirmed that the interspersed RAB5C signals were co-localized with TBC1D21 within the midpieces of the mature sperm cells. Conclusion: Based on these results, we suggest RAB5C, a TBC1D21 interactor, was involved in acrosomal and midpiece formation.