Functional role of the hCNGB3 in regulation of human cone CNG channel: effect of rod monochromacy-associated mutations in hCNGB3 on channel function

Abstract

[PURPOSE] The human cone photoreceptor CNG channel is thought to comprise hCNGA3- and hCNGB3-subunits. The purpose was to examine functional role of hCNGB3 in modulation of human cone CNG channels and to characterize functional consequences of rod monochromacy-associated mutations in hCNGB3 (S435F and D633G). [METHODS] Macroscopic patch currents were recorded from HEK293 cells expressing homomeric and heteromeric channels using inside-out patch-clamp technique. [RESULTS] Both hCNGA3 homomeric and hCNGA3/hCNGB3 heteromeric channels were activated by cGMP, with K_1/2 of 11.1±1.0 and 26.2±1.9 μM, respectively. The hCNGA3 channels appeared to be more sensitive to inhibition by extracellular Ca²⁺ compared with hCNGA3/hCNGB3 channels. Coexpression of either of rod monochromacy-associated mutants of hCNGB3 with hCNGA3 significantly reduced K_1/2 value for cGMP but little affected the sensitivity to extracellular Ca²⁺, compared with wild-type heteromeric channels. The selectivity of homomeric and heteromeric channels for monovalent cations was largely similar. Immunoprecipitation experiments showed association of hCNGA3-subunit with both of wild-type and mutant hCNGB3-subunits. [CONCLUSIONS] The rod monochromacy-associated S435F and D633G mutations in hCNGB3 evokes a significant increase in the apparent affinity for cGMP, which should alter cone function and thereby contribute at least partly to pathogenesis of the disease. [Jpn J Physiol 54 Suppl:S136 (2004)]