Certain membrane channels including acetylcholine receptors, gap junction (GJ) channels, and aquaporins arrange into large clusters in the plasma membrane (PM). However, how these channels are recruited to the clusters is unknown. To address this question, we have investigated delivery of GJ channel subunits (connexons) assembled from green fluorescent protein (GFP)-tagged connexin 43 (Cx43) to the PM and GJs in living cells. Fluorescence-photobleaching of distinct areas of Cx43-GFP GJs demonstrated that newly synthesized channels were accrued to the outer margins of channel clusters. Time-lapse microscopy further revealed that connexons were delivered in vesicular carriers traveling along microtubules from the Golgi to the PM. Routing and insertion of connexons occurred predominantly into the nonjunctional PM. These PM connexons can move laterally as shown by photobleaching and thus, can reach the margins of channel clusters. There, the apposing PMs are close enough to allow connexons to dock into complete GJ channels. When connexon delivery to the PM was inhibited by brefeldin A, or nocodazole pretreatment, the PM pool initially enabled connexon accrual to the clusters but further accrual was inhibited upon depletion. Taken together, our results indicate that GJ channel clusters grow by accretion at their outer margins from connexon subunits that were delivered to the nonjunctional PM, and explain how connexons in the PM can function in intra-͞extracellular signaling before GJ channel formation and direct cell-cell communication.Cx43 ͉ GFP ͉ photo-bleaching ͉ secretion ͉ time-lapse microscopy

Beyond its well-documented role in vesicle endocytosis, clathrin has also been implicated in the internalization of large particles such as viruses, pathogenic bacteria, and even latex beads. We have discovered an additional clathrin-dependent endocytic process that results in the internalization of large, double-membrane vesicles at lateral membranes of cells that are coupled by gap junctions (GJs). GJ channels bridge apposing cell membranes to mediate the direct transfer of electrical currents and signaling molecules from cell to cell. Here, we report that entire GJ plaques, clusters of GJ channels, can be internalized to form large, double-membrane vesicles previously termed annular gap junctions (AGJs). These internalized AGJ vesicles subdivide into smaller vesicles that are degraded by endo/lysosomal pathways. Mechanistic analyses revealed that clathrin-dependent endocytosis machinery-components, including clathrin itself, the alternative clathrin-adaptor Dab2, dynamin, myosin-VI, and actin are involved in the internalization, inward movement, and degradation of these large, intercellular double-membrane vesicles. These findings contribute to the understanding of clathrin's numerous emerging functions. INTRODUCTIONThe role of clathrin in endocytosis is well documented. This protein forms a typical curved lattice around endocytic vesicles that are internalized at the plasma membrane (PM). In addition, the involvement of clathrin in several uncharacteristic endocytic processes has been reported, including the internalization of viruses, pathogenic bacteria, and large latex beads (Aggeler and Werb, 1982;Ehrlich et al., 2004;Rust et al., 2004;Veiga and Cossart, 2005). Here, we describe another function of the clathrin-dependent endocytic machinery that results in the internalization of large, doublemembrane vesicles at lateral PMs of cells that are coupled by gap junctions (GJs).GJs are ubiquitously distributed channels that connect the cytoplasms of two apposing cells each participating in this connection via a half channel termed a connexon to provide direct cell-to-cell communication. Connexons are hexamers of four-pass membrane proteins called connexins (Cxs;Bruzzone et al., 1996;Kumar and Gilula, 1996). Once transported to the PM, GJ channels cluster into two-dimensional arrays termed plaques that can be composed of a few to many thousands of individual channels and vary from a few square nanometers to many square micrometers (Bruzzone et al., 1996; Falk, 2000a;Severs et al., 2001). GJ channels can open and close (gate) and physiological parameters, including intracellular pH, Ca 2ϩ concentration, and Cx phosphorylation, are known to modulate GJ channel gating and the extent of GJ-mediated intercellular coupling (Delmar et al., 2004;Lampe and Lau, 2004;Moreno, 2005). However, the extent of intercellular coupling could also be regulated through altering the number of GJ channels in the PM.Cxs have a surprisingly short half-life of only 1-5 h, leading to a rapid GJ and Cx protein turnover (Fallon and Goodeno...

Gap junctions are specialized cell-cell junctions that mediate intercellular communication. They are composed of connexin proteins, which form transmembrane channels for small molecules [1, 2]. The C-terminal tail of connexin-43 (Cx43), the most widely expressed connexin member, has been implicated in the regulation of Cx43 channel gating by growth factors [3-5]. The Cx43 tail contains various protein interaction sites, but little is known about binding partners. To identify Cx43-interacting proteins, we performed pull-down experiments using the C-terminal tail of Cx43 fused to glutathione-S-transferase. We find that the Cx43 tail binds directly to tubulin and, like full-length Cx43, sediments with microtubules. Tubulin binding to Cx43 is specific in that it is not observed with three other connexins. We established that a 35-amino acid juxtamembrane region in the Cx43 tail, which contains a presumptive tubulin binding motif, is necessary and sufficient for microtubule binding. Immunofluorescence and immunoelectron microscopy studies reveal that microtubules extend to Cx43-based gap junctions in contacted cells. However, intact microtubules are dispensable for the regulation of Cx43 gap-junctional communication. Our findings suggest that, in addition to its well-established role as a channel-forming protein, Cx43 can anchor microtubule distal ends to gap junctions and thereby might influence the properties of microtubules in contacted cells.

Gap junctions (GJs) are the only known cellular structures that allow a direct transfer of signaling molecules from cell-to-cell by forming hydrophilic channels that bridge the opposing membranes of neighboring cells. The crucial role of GJ-mediated intercellular communication (GJIC) for coordination of development, tissue function, and cell homeostasis is now well documented. In addition, recent findings have fueled the novel concepts that connexins, although redundant, have unique and specific functions, that GJIC may play a significant role in unstable, transient cell-cell contacts, and that GJ hemi-channels by themselves may function in intra-/extracellular signaling. Assembly of these channels is a complicated, highly regulated process that includes biosynthesis of the connexin subunit proteins on endoplasmic reticulum membranes, oligomerization of compatible subunits into hexameric hemi-channels (connexons), delivery of the connexons to the plasma membrane, head-on docking of compatible connexons in the extracellular space at distinct locations, arrangement of channels into dynamic, spatially and temporally organized GJ channel aggregates (so-called plaques), and coordinated removal of channels into the cytoplasm followed by their degradation. Here we review the current knowledge of the processes that lead to GJ biosynthesis and degradation, draw comparisons to other membrane proteins, highlight novel findings, point out contradictory observations, and provide some provocative suggestive solutions.