Identification of SAMT family proteins as substrates of MARCH11 in mouse spermatids |
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Authors: | Keiichiro Yogo Hidehiro Tojima Jun-ya Ohno Takuya Ogawa Nobuhiro Nakamura Shigehisa Hirose Tatsuo Takeya Tetsuya Kohsaka |
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Institution: | (1) Animal Reproduction and Physiology, Faculty of Agriculture, Shizuoka University, 836 Ohya, Suruga, Shizuoka 422-8529, Japan;(2) Department of Biological Sciences, Tokyo Institute of Technology, Yokohama 226-8501, Japan;(3) Graduate School of Biological Sciences, Nara Institute of Science and Technology, Ikoma 630-0192, Japan |
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Abstract: | MARCH11, a RING-finger transmembrane ubiquitin ligase, is predominantly expressed in spermatids and localized to the trans-Golgi
network (TGN) and multivesicular bodies (MVBs). Because ubiquitination acts as a sorting signal of cargo proteins, MARCH11
has been postulated to mediate selective protein sorting via the TGN–MVB pathway. However, the physiological substrate of
MARCH11 has not been identified. In this study, we have identified and characterized SAMT1, a member of a novel 4-transmembrane
protein family, which consists of four members. Samt1 mRNA and its expression product were found to be specific to the testis and were first detected in germ cells 25 days after
birth in mice. Immunohistochemical analysis further revealed that SAMT1 was specifically expressed in haploid spermatids during
the cap and acrosome phases. Confocal microscopic analysis showed that SAMT1 co-localized with MARCH11 as well as with fucose-containing
glycoproteins, another TGN/MVB marker, and LAPM2, a late endosome/lysosome marker. Furthermore, we found that MARCH11 could
increase the ubiquitination of SAMT1 and enhance its lysosomal delivery and degradation in an E3 ligase activity-dependent
manner. In addition, the C-terminal region of SAMT1 was indispensable for its ubiquitination and proper localization. The
other member proteins of the SAMT family also showed similar expression profile, intracellular localization, and biochemical
properties, including ubiquitination by MARCH11. These results suggest that SAMT family proteins are physiological substrates
of MARCH11 and are delivered to lysosomes through the TGN–MVB pathway by a ubiquitin-dependent sorting system in mouse spermatids. |
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