Helical wheel representation of reactive residues at the TM1/E1 border of Cx46 and Cx32*Cx43E1. (A) Helical wheel representation of residues R33 through G46 of Cx46. The residues span the TM1/E1 border, with residue E42 believed to be located on the membrane border. Residues accessible to MTS reagents applied to both the intercellular and extracellular face of the unapposed hemichannel are shown in red. These residues reside on one side of the helix, subtend an arc of ∼80°, and line the pore of the open Cx46 unapposed hemichannel. Residue R33 is topmost in the figure, with the residue number increasing counterclockwise. Data are taken from Kronengold et al. (2003). (B) Helical wheel representation of homologous residues R32 through G45 of Cx32*Cx43E1 that line the pore of the open unapposed Cx32*Cx43E1 channel. Residues examined in this study are marked with asterisks. Two residues, V38C and G45C, react with MTSEA–biotin-X when the reagent is applied from either the intracellular or extracellular face of the channel and consequently are assigned as pore lining. Expression levels of S42C and E41C were insufficient to allow studies of accessibility to thiol modifying reagents, whereas W44C did not express membrane current. These residues are marked by two asterisks. (C) Helical wheel representation of the loop gate closed Cx32*Cx43E1 channel. The helical wheel pictured in B was rotated 140° clockwise to position residues A40C and A43C. This position accounts for the observed sensitivity of residues A40C and A43C to Cd²⁺ and their modification by bBBr when the channel is closed by loop gating (see Discussion).