Title : Membrane topology of the amiloride-sensitive epithelial
sodium channel
Abstract :
- The amiloride-sensitive epithelial sodium channel ( ENaC ) is involved in fluid and electrolyte absorption across a number of epithelia, and cloning of several ENaC subunits has begun to facilitate investigation of the structure, function, and regulation of this channel
- Analysis of the amino acid sequence has revealed two potential membrane-spanning domains , but little else is known about the structure of ENaC
- To investigate the membrane topology of one subunit , alpha r ENaC , we used in vitro transcription, translation, and translocation into microsomal membranes
- This generated a glycosylated protein of 93 kDa
- Sequence analysis also revealed eight potential sites for N-glycosylation, six of which were found to be glycosylated ( Asn190, Asn259, Asn320, Asn339, Asn424, and Asn538 ), indicating that they are extracellular
- The C terminus was localized as intracellular based on antibody recognition and protease sensitivity of a tagged epitope at the C terminus
- The N terminus was also found to be intracellular, based on its protease sensitivity
- Similar results were obtained by expression in Xenopus oocytes
- Together, these results support a model of alpha r ENaC consisting of an intracellular N terminus and C terminus , a large N-glycosylated extracellular domain , and two membrane-spanning domains that each pass once through the plasma membrane
- Because of their sequence similarity, it is likely that this structure is shared by other ENaC subunits and possibly the degenerins of Caenorhabditis elegans as well