RFPL4 interacts with oocyte proteins of the ubiquitin-proteasome degradation pathway

Oocyte meiosis and early mitotic divisions in developing embryos rely on the timely production of cell cycle regulators and their clearance via proteasomal degradation. Ret Finger Protein-Like 4 (Rfpl4), encoding a RING finger-like protein with a B30.2 domain, was discovered during an in silico search for germ cell-specific genes. To study the expression and functions of RFPL4 protein, we performed immunolocalizations and used yeast two-hybrid and other protein-protein interaction assays. Immunohistochemistry and immunofluorescence showed that RFPL4 accumulates in all growing oocytes and quickly disappears during early embryonic cleavage. We used a yeast two-hybrid model to demonstrate that RFPL4 interacts with the E2 ubiquitin-conjugating enzyme HR6A, proteasome subunit beta type 1, ubiquitin B, as well as a degradation target protein, cyclin B1. Coimmunoprecipitation analyses of in vitro translated proteins and extracts of transiently cotransfected Chinese hamster ovary (CHO)-K1 cells confirmed these findings. We conclude that, like many RING-finger containing proteins, RFPL4 is an E3 ubiquitin ligase. The specificity of its expression and these interactions suggest that RFPL4 targets cyclin B1 for proteasomal degradation, a key aspect of oocyte cell cycle control during meiosis and the crucial oocyte-to-embryo transition to mitosis.