Phosphorylation-Regulated Endoplasmic Reticulum Retention Signal in the Renal Outer-Medullary K+ Channel (ROMK)

The renal outer-medullary K+ channel (ROMK; Kir1.1) mediates K+ secretion in the renal mammalian nephron that is critical to both sodium and potassium homeostasis. The posttranscriptional expression of ROMK in the plasma membrane of cells is regulated by delivery of protein from endoplasmic reticulum (ER) to the cell surface and by retrieval by dynamin-dependent endocytic mechanisms in clathrin-coated pits. The S44 in the NH2 terminus of ROMK1 can be phosphorylated by PKA and serum- and glucocorticoid-inducible kinase-1, and this process increases surface expression of functional channels. We present evidence that phosphorylation of S44 modulates channel expression by increasing its cell surface delivery consequent to suppression of a COOH-terminal ER retention signal. This phosphorylation switch of the ER retention signal could provide a pool of mature and properly folded channels for rapid delivery to the plasma membrane. The x-ray crystal structures of inward rectifier K+ channels have shown a close apposition of the NH2 terminus with the distal COOH terminus of the adjacent subunit in the channel homotetramer, which is important to channel gating. Thus, NH2-terminal phosphorylation modifying a COOH-terminal ER retention signal in ROMK1 could serve as a checkpoint for proper subunit folding critical to channel gating.