Pathways and control of connexin oligomerization

Abstract

Connexins form gap junction channels that link neighboring cells into an intercellular communication network. Many cells that express multiple connexins produce heteromeric channels containing at least two connexins, which provides a means to fine tune gap junctional communication. Formation of channels by multiple connexins is controlled at two levels: by inherent structural compatibilities that enable connexins to hetero-oligomerize and by cellular mechanisms that restrict the formation of heteromers by otherwise compatible connexins. Here, I discuss roles for secretory compartments beyond the endoplasmic reticulum in connexin oligomerization and evidence that suggests that membrane microdomains help regulate connexin trafficking and assembly.

Figure 1

Connexin homology and structure. (a) Phylogram of 20 human connexins calculated using ClustalW and omitting C-terminal domains . By protein homology, connexins form two major subgroups, α and β, with an additional group of connexins with intermediate homology . Different connexins are denoted by Cx plus a number corresponding to the predicted molecular mass based on the amino acid sequence. Mouse connexin names that differ from their human orthologs are shown in parentheses. (b) Line diagram corresponding to an individual connexin. The two extracellular loop domains are interconnected by three disulfide bridges. Green denotes regions with high amino acid sequence homology within the entire connexin protein family, yellow denotes regions in which amino acids are homologous within a subset of connexins and gray denotes divergent regions of connexins that vary in amino acid sequence and size. Numbers correspond to positions of Cx43 amino acids that define different classes of connexin homology domains.

Figure 2

Gap junction channels. (a)En face view of a connexin hemichannel, showing only the transmembrane helical domains as viewed from the outside of the cell looking into the permeability pore (*). The image was generated with RasMol using coordinates from . In this model, transmembrane (TM) domains correspond to TM1 (blue), TM2 (aqua), TM3 (yellow) and TM4 (red). (b) Types of gap junction channel. Ovals represent individual connexins and the dashed line indicates the path of the aqueous portion of the channel. Shown from left to right are a homomeric gap junction channel composed of a single type of connexin, a heterotypic channel composed of two different connexins expressed by two adjacent cells and a heteromeric channel composed of two different connexins co-expressed in the same cell.

Figure 3

Cellular control of connexin oligomerization. Immunofluorescence images of (a) osteoblastic cells and (b) type I alveolar epithelial cells are shown, both of which express endogenous Cx43 (red) and Cx46 (green). (a) Osteoblastic cells prevent Cx43 and Cx46 from intermixing. Only Cx43 is targeted by these cells to assemble into gap junctions at the plasma membrane (red). However, Cx46 is retained by osteoblastic cells in the TGN (green perinuclear fluorescence). (b) By contrast, type I alveolar epithelial cells show co-localization of Cx43 (red) and Cx46 (green) to produce yellow fluorescence. Formation of Cx43–Cx46 heteromers was demonstrated by co-immunopurification. Adapted, with permission, from . Scale bar=10 μm.

Figure 4

Connexin oligomerization pathways. Connexins are co-translationally inserted into the ER membrane. Depending on the connexin subtype, oligomerization can occur either in the ERGIC (Cx32, blue) or the TGN (Cx43, green). The potential for connexin oligomerization in the ER, driven by high levels of connexin expression, is also shown for Cx32. Hemichannels are subsequently transported to the plasma membrane, where they can function as channels or pair with hemichannels on adjacent cells to form complete intercellular channels. Channels at the plasma membrane further assemble into semi-crystalline arrays known as gap junction plaques, which can contain from tens to thousands of channels. Homogenous plaques are composed of either a single connexin or heteromeric connexins (not shown). Heterogeneous plaques contain regions enriched for different connexins.