Ropporin, a sperm-specific binding protein of rhophilin, that is localized in the fibrous sheath of sperm flagella

Akiko Fujita, Ken Ichi Nakamura, Takayuki Kato, Naoki Watanabe, Toshimasa Ishizaki, Kazushi Kimura, Akira Mizoguchi, Shuh Narumiya

    Research output: Contribution to journalArticlepeer-review

    122 Citations (Scopus)


    The small GTPase Rho functions as a molecular switch that regulates various cellular processes such as cell adhesion, motility, gene expression and cytokinesis. We previously isolated several putative Rho targets including rhophilin which bound selectively to the GTP-bound form of Rho. Rhophilin is expressed highly in testis and is localized specifically in sperm flagella. The presence of a PDZ domain at the carboxy terminus of rhophilin suggested that rhophilin works as an adaptor molecule. To test this hypothesis, we employed a yeast two hybrid system using the rhophilin PDZ domain as a bait, and screened a mouse testis cDNA library. We isolated several positive clones containing the same insert. The open reading frame of the cDNA encoded a navel protein of 212 amino acids designated as ropporin from a Japanese word 'oppo' (the tail). The amino-terminal 40 amino acid sequence of ropporin showed high homology to that of the regulatory subunit of type II cAMP-dependent protein kinase, which is involved in dimerization and binding to A-kinase anchoring proteins, Consistently, a yeast two hybrid assay and gel filtration of recombinant ropporin indicated that ropporin dimerizes through this domain. Deletion analysis indicated that the carboxy-terminal four amino acids are essential for binding of ropporin to rhophilin, and ropporin and RhoV14 coprecipitated in the presence of rhophilin in vitro. Northern blot analysis showed that ropporin is exclusively expressed in testis, and induced at the late stage of spermatogenesis. This induction paralleled that of rhophilin. Immunocytochemistry using anti-ropporin antibody showed that ropporin is localized in the principal piece and the end piece of sperm flagella. Electronmicroscopy revealed that ropporin is mostly localized in the inner surface of the fibrous sheath while rhophilin is present in the outer surface of the outer dense fiber. These results suggest that rhophilin and ropporin may form a complex in sperm flagella.

    Original languageEnglish
    Pages (from-to)103-112
    Number of pages10
    JournalJournal of cell science
    Issue number1
    Publication statusPublished - 2000 Feb 7


    • Fibrous sheath
    • Rho
    • Rhophilin
    • Ropporin

    ASJC Scopus subject areas

    • Cell Biology


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