Lipid Rafts Prepared by Different Methods Contain Different Connexin Channels, but Gap Junctions Are Not Lipid Rafts

Abstract

Cell extraction with cold nonionic detergents or alkaline carbonate prepares an insoluble membrane fraction whose buoyant density permits its flotation in discontinuous sucrose gradients. These lipid "rafts" are implicated in protein sorting and are attractive candidates as platforms that coordinate signal transduction pathways with intracellular substrates. Gap junctions form a direct molecular signaling pathway by end-to-end apposition of hemichannels containing one (homomeric) or more (heteromeric) connexin isoforms. Residency of channels composed of Cx26 and/or Cx32 in lipid rafts was assessed by membrane insolubility in alkaline carbonate or different concentrations of Triton X100, Nonidet P40 and Brij-58 nonionic detergents. Using Triton X100, insoluble raft membranes contained homomeric Cx32 channels, but Cx26-containing channels only when low detergent concentrations were used. Results were similar using Nonidet P40, except that Cx26-containing channels were excluded from raft membranes at all detergent concentrations. In contrast, homomeric Cx26 channels were enriched within Brij-58-insoluble rafts, whereas Cx32-containing channels partitioned between raft and nonraft membranes. Immunofluorescence microscopy showed prominent colocalization only of nonjunctional connexin channels with raft plasma membrane; junctional plaques were not lipid rafts. Rafts prepared by different extraction methods had considerable quantitative and qualitative differences in their lipid compositions. That functionally different nonjunctional connexin channels partition among rafts with distinct lipid compositions suggests that unpaired Cx26 and/or Cx32 channels exist in membrane domains of slightly different physicochemical character. Rafts may be involved in trafficking of plasma membrane connexin channels to gap junctions.