Recently, we identified a cell-surface marker protein, TEX101, that is unique to male and female germ cells. On/off switching of TEX101 expression in germ cells is closely linked to the kinetics of gametogenesis. In the present study, we isolated testicular proteins by immunoprecipitation with anti-TEX101 antibody and identified the proteins using liquid chromatography/tandem mass spectrometry. Of three proteins identified (annexin 2, ly6k, and cellubrevin), a biochemical association between TEX101 and cellubrevin was confirmed by immunoprecipitation-Western blotting experiments. Immunohistocheniistry using a cellubrevin-specific antibody indicated that the molecule is abundant on spermatocytes and early-stage spermatids, whereas negligible amounts are found in Sertoli cells, spermatogonia, spermatozoa, and late-stage spermatids. Most of the intracellular cellubrevin appeared to be juxtaposed with intracellular TEX101, and membrane-associated cellubrevin was docked near TEX101-positive plasma membranes on the cytoplasmic side. This close association was never observed on the outer surface of the plasma membrane. From these results we concluded that cellubrevin-dependent membrane trafficking is involved in TEX101-transport to the surface of male-germ cells. (c) 2006 Elsevier Inc. All rights reserved.