Functional maturation of cyclic-GMP gated ion channel protein requires an intact S4 structural motif.

FAILLACE MP, BERNABEU R AND KORENBROT J

JOURNAL OF BIOLOGICAL CHEMISTRY

AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC

Año: 2004 vol. 279 p. 22643 - 22653

0021-9258

We examined cellular protein processing and functionalexpression of photoreceptor cyclic nucleotidegated(CNG) ion channels. In a mammalian cell line, wildtype bovine cone photoreceptor channel subunits(bCNGA3) convert from an unglycosylated state, at 90kDa, to two glycosylated states at 93 and 102 kDa as theytransit within the cell to their final location at theplasma membrane. Glycosylation per se is not requiredto yield functional channels, yet it is a milestone thatdistinguishes sequential steps in channel protein maturation.CNG ion channels are not gated by membranevoltage although their structure includes the transmembraneS4 motif known to function as the membrane voltagesensor in all voltage-gated ion channels. S4 must befunctionally important because its natural mutation incone photoreceptor CNG channels is associated withachromatopsia, a human autosomal inherited loss ofcone function. Point mutation of specific, not all,charged and neutral residues within S4 cause failure offunctional channel expression. Cellular channel proteinprocessing fails in every one of the non-functional S4mutations we studied. Mutant proteins do not reach the102-kDa glycosylated state and do not arrive at theplasma membrane. They remain trapped within the endoplasmicreticulum and fail to transit out to the Golgiapparatus. Coexpression of cone CNG subunit(CNGB3) does not rescue the consequence of S4 mutationsin CNGA3. It is likely that an intact S4 is requiredfor proper protein folding and/or assembly in the endoplasmicreticulum membrane.