Identification of a unique endoplasmic retention motif in the Xenopus GIRK5 channel and its contribution to oocyte maturation

G protein‐activated inward‐rectifying potassium (K+) channels (Kir3/GIRK) participate in cell excitability. The GIRK5 channel is present in Xenopus laevis oocytes. In an attempt to investigate the physiological role of GIRK5, we identified a noncanonical di‐arginine endoplasmic reticulum (ER) retention motif (KRXY). This retention motif is located at the N‐terminal region of GIRK5, coded by two small exons found only in X. laevis and X. tropicalis. These novel exons are expressed through use of an alternative transcription start site. Mutations in the sequence KRXY produced functional channels and induced progesterone‐independent oocyte meiotic progression. The chimeric proteins enhanced green fluorescent protein (EGFP)‐GIRK5‐WT and the EGFP‐GIRK5K13AR14A double mutant, were localized to the ER and the plasma membrane of the vegetal pole of the oocyte, respectively. Silencing of GIRK5 or blocking of this channel by external barium prevented progesterone‐induced meiotic progression. The endogenous level of GIRK5 protein decreased through oocyte stages in prophase I augmenting by progesterone. In conclusion, we have identified a unique mechanism by which the expression pattern of a K+ channel evolved to control Xenopus oocyte maturation. The Xenopus GIRK5 potassium channel is orthologous to human GIRK4. The phosphorylatable motif KRXY at the N‐terminus retains GIRK5 at the endoplasmic reticulum. This retention motif is contained in one of two small exons found only in X. laevis and X. tropicalis. Any mutation or dephosphorylation of tyrosine in KRXY allows delivery of GIRK5 to the plasma membrane at the vegetal pole, promoting oocyte maturation. This unique Xenopus retention motif was acquired late in evolution.