Structure-function studies of mutations of vertebrate gap junction connexin 32 proteins associated with the x-linked form of Charcot -Marie -Tooth disease
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Connexin32 (Cx32) is a 32-kilodalton (kDa) transmembrane protein expressed most abundantly in the liver, and also in the kidney, intestines, lung, spleen, stomach, testes, and Schwann cells.;I used Xenopus laevis oocytes to transiently express mutant Cx32 proteins and applied the dual whole-cell voltage-clamp technique to functionally characterize them. Our structure-function studies of Cx32 are focused on (1) the characterization of ten mutations---I30N, M34T, V35K V38K T86A, T86N, P87A, E102G Delta111--116, and G12S---identified in the Cx32 gene of patients suffering the X-linked form of Charcot-Marie-Tooth (CMTX) disease, (2) the examination of the role of T86 and P87 residues as forming a proline kink (PK) region in the second transmembrane domain (TM2) that may function as a dynamic element in the voltage-dependent gating of vertebrate gap junction channels, and (3) the identification of the molecular determinants responsible for the differences in the voltage-dependent phenotype of the P87G mutation in the Cx26P87G and Cx32P87G mutants.;Nine Cx32 CMTX mutations---130N, M34T, V35M, V38M, T86A, T86N, P87A, E102G, and Delta111--116---produced functional homotypic channels, seven of which behave differently from wild type Cx32 in various degrees. Only the G12S mutant failed to show any functional expression.;Proline kink (PK) regions are highly conserved structural motifs found in the of transmembrane domains of a variety of transmembrane proteins. I have created two sets of mutations to test whether the PK motif functions as a dynamic element. The first set of mutations was targeted at P87 to create a series of amino acids that are predicted to confer increasing degrees of alpha-helical character to TM2 (G ≤ A < V), and therefore increase functional disturbance. The second set of mutations were directed at residues that are adjacent to P87, namely T86 and A88, and designed to preserve the PK region while targeting its structural flexibility. I propose that the PK region can mediate conformational changes between the open and closed conformational states of Cx32 channels through modulation of the hydrogen bonding potential of the T86 residue. Finally, I have explored the differences in the voltage-dependent gating effects of the P87G mutation between the Cx26 and Cx32. (Abstract shortened by UMI.).
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