Dynamic trafficking and delivery of connexons to the plasma membrane and accretion to gap junctions in living cells

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Abstract
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 photo-bleaching 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.