The gamma subunit of Na/K-ATPase: an exceptional, small transmembrane protein.

The gamma subunit has been characterized as a fine-tuning modulator of the Na/K-ATPase expressed in kidney tissues. This small single transmembrane domain protein interacts with the alpha subunit of Na/K-ATPase to increase affinity for ATP and decrease affinity for Na allowing medullary cells to continue pump activity under reduced cellular ATP levels. The gamma subunit is undetectable in kidney cell cultures grown under isotonic conditions and expression is induced with acute or chronic exposure to hypertonicity. The gamma subunit demonstrates remarkable regulatory complexity including induction by chloride ions rather than sodium, the differential expression of at least 2 isoforms, involvement of separate MAP kinase signaling pathways for transcription (JNK) and translation (PI3K) as well as cell type regulation of expression. Mutation in the transmembrane domain of the gamma subunit has been implicated in cases of primary hypomagnesemia. Alternative roles have been established for the gamma subunit in embryonic development and quite possibly additional functions in cell signaling as yet unrecognized may be possible.