Ropporin 1 (ROPN1)

The N-terminal 40-amino acid sequence of Ropn1 showed high homology to that of the regulatory subunit of type II cAMP-dependent protein kinase. Yeast 2-hybrid assay and gel filtration indicated that Ropn1 homodimerizes through this domain. Deletion analysis showed that the C-terminal 4 amino acids are essential for binding of ropporin to rhophilin. Ropn1 and Rho coprecipitated in the presence of rhophilin in vitro. Immunocytochemistry showed that ropporin was localized in the principal piece and the end piece of sperm flagella. Electron microscopy revealed that ropporin was mostly localized in the inner surface of the fibrous sheath, while rhophilin was present in the outer surface of the outer dense fiber. These results suggested that rhophilin and ropporin may form a complex in sperm flagella.