The intracellular domains of connexins are essential for the assembly of gap junctions. For connexin 36 (Cx36), the major neuronal connexin, it has been shown that a dysfunctional PDZ binding motif interferes with electrical synapse formation. However, it is still unknown how this motif coordinates the transport of Cx36. In the present study, we characterize a phenotype of Cx36 mutants that lack a functional PDZ binding motif using HEK293T cells as an expression system. We provide evidence that an intact PDZ binding motif is critical for proper ER export of Cx36. Removing the PDZ binding motif of Cx36 results in ER retention and the formation of multi-membrane vesicles containing gap junction-like connexin aggregates. Using a combination of site directed mutagenesis and electron micrographs we reveal that these vesicles consist of Cx36 channels that docked prematurely in the ER. Our data suggest a model in which ER-retained Cx36 channels reshape the ER membrane into concentric whorls that are released into the cytoplasm.

Ca2+/calmodulin-dependent protein kinase II (CaMKII) binding and phosphorylation of mammalian connexin-36 (Cx36) potentiate electrical coupling. To explain the molecular mechanism of how Cx36 modifies plasticity at gap junctions, we investigated the roles of ionotropic N-methyl-D-aspartate receptors and pannexin1 (Panx1) channels in regulating Cx36 binding to CaMKII. Pharmacological interference and site-directed mutagenesis of protein interaction sites shows that NMDA receptor activation opens Cx36 channels, causing the Cx36- CaMKII binding complex to adopt a compact conformation. Ectopic Panx1 expression in a Panx1 knock-down cell line is required to restore CaMKII mediated opening of Cx36. Furthermore, blocking of Src-family kinase activation of Panx1 is sufficient to prevent the opening of Cx36 channels. Our research demonstrates that the efficacy of Cx36 channels requires convergent calcium-dependent signaling processes in which activation of ionotropic N-methyl-D-aspartate receptor, Src-family kinase, and Pannexin1 open Cx36. Our results add to the best of our knowledge a new twist to mounting evidence for molecular communication between these core components of electrical and chemical synapses.

The gap junctional protein connexin 36 (Cx36) has been co-purified with the lipid raft protein caveolin-1 (Cav-1). The relevance of an interaction between the two proteins is unknown. In this study, we explored the significance of Cav-1 interaction in the context of intracellular and membrane transport of Cx36. Coimmunoprecipitation assays and Förster resonance energy transfer analysis (FRET) were used to confirm the interaction between the two proteins in the Neuro 2a cell line. We found that the Cx36 and Cav-1 interaction was dependent on the intracellular calcium levels. By employing different microscopy techniques, we demonstrated that Cav-1 enhances the vesicular transport of Cx36. Pharmacological interventions coupled with cell surface biotinylation assays and FRET analysis revealed that Cav-1 regulates membrane localization of Cx36. Our data indicate that the interaction between Cx36 and Cav-1 plays a role in the internalization of Cx36 by a caveolin-dependent pathway.

Pannexin 1 (Panx1) is a ubiquitously expressed hexameric integral membrane protein known to function as an adenosine triphosphate (ATP) release channel. Panx1 proteins exist in unglycosylated core form (Gly0). They undergo critical post-translational modifications forming the high mannose glycosylation state (Gly1) in the endoplasmic reticulum (ER) and the complex glycosylation state (Gly2) in the Golgi apparatus. The regulation of transition from the ER to the cell membrane is not fully understood. Using site-specific mutagenesis, dye uptake assays, and interaction testing, we identified two conserved aromatic residues, Trp123 and Tyr205, in the transmembrane domains 2 and 3 of the zebrafish panx1a protein. Results suggest that both residues primarily govern the assembly of panx1a subunits into channels, with mutant proteins failing to interact. The results provide insight into a mechanism enabling regulation of Panx1 oligomerization, glycosylation, and trafficking.Biomolecules 2020, 10, 272 2 of 15 teleost whole-genome duplication event that occurred between 320 and 350 million years ago, a panx1a ohnologue exists (panx1b) [16]. Panx1a and panx1b show distinct tissue expressions, glycosylation patterns, and electrophysiological gating properties [14]. This study will focus on the panx1a ohnologue.Electrophysiological gating properties and multiple factors regulating the Panx1 channel, as well as complex pharmacology, have been described [17,18]. Panx1 blockers include carbenoxolone, mefloquine, and flufenamic acid, which also act on gap junction proteins. Potassium and glutamate can activate pannexins. Studies across multiple fields demonstrated that Panx1 is a major molecular hub interacting with many signaling pathways. To address the complex life cycle of Panx1, we explored the role of an aromatic-aromatic interaction between amino acids W123 and Y205 in the cytoplasmic loop of panx1a near transmembrane (TM) domains 2 and 3, respectively. Aromatic-aromatic interactions have been previously shown to be important in TM-TM association of membrane proteins [19,20]. The forces of these interactions help strengthen oligomerization and suggest a role in folding and stabilization. Both W123 and Y205 are highly conserved between various membrane channels and gap junction proteins. Mutation analysis of panx1a paired with co-localization and protein interaction studies led to the conclusion that the two aromatic residues are vital for the structural stabilization and interaction of panx1a TM domains before insertion into the cell membrane. Outcomes of this study are relevant to understand how Panx1 proteins mature and traffic to the cell membrane.

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