Crucial motifs and residues in the extracellular loops influence the formation and specificity of connexin docking

Abstract

Most of the early studies on gap junction (GJ) channel function and docking compatibility were on rodent connexins, while recent research on GJ channels gradually shifted from rodent to human connexins largely due to the fact that mutations in many human connexin genes are found to associate with inherited human diseases. The studies on human connexins have revealed some key differences from those found in rodents, calling for a comprehensive characterization of human GJ channels. Functional studies revealed that docking and formation of functional GJ channels between two hemichannels are possible only between docking-compatible connexins. Two groups of docking-compatible rodent connexins have been identified. Compatibility is believed to be due to their amino acid residue differences at the extracellular loop domains (E1 and E2). Sequence alignment of the E1 and E2 domains of all connexins known to make GJs revealed that they are highly conserved and show high sequence identity with human Cx26, which is the only connexin with near atomic resolution GJ structure. We hypothesize that different connexins have a similar structure as that of Cx26 at the E1 and E2 domains and use the corresponding residues in their E1 and E2 domains for docking. Based on the Cx26 GJ structure and sequence analysis of well-studied connexins, we propose that the E1-E1 docking interactions are staggered with each E1 interacting with two E1s on the docked connexon. The putative E1 docking residues are conserved in both docking-compatible and -incompatible connexins, indicating that E1 does not likely serve a role in docking compatibility. However, in the case of E2-E2 docking interactions, the putative docking residues are only conserved within the docking-compatible connexins, suggesting the E2 is likely to serve the function of docking compatibility. Docking compatibility studies on human connexins have attracted a lot of attention due to the fact that putative docking residues are mutational hotspots for several connexin-linked human diseases. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Connexins in the control of vasomotor function

“…The extracellular loops contain the pairing regions by which the connexins couple with their counterparts in the membrane of an adjacent cell. 24 In contrast, the intermediate loop as well as the N-and C-terminal ends of the protein (also called amino tails and carboxyl tails) are located on the cytoplasmic side. Whereas the sequences of the N-termini as well as the extracellular loops are highly conserved among connexins, the intracellular loops and, especially, the C-termini are more variable.…”

Heterotypic docking compatibility of human connexin37 with other vascular connexins

“…These groups became part of their genetic nomenclature Kumar and Gilula, 1996). As extensively discussed in the review (Bai et al, 2018), there are a few differences to note between mouse and human connexins apart from their predicted function to form intercellular channels. First, for some connexins genes, no homologues were identified between the two species such as mCx33 and hCx25.…”

Connexin 46 and connexin 50 gap junction channel open stability and unitary conductance are shaped by structural and dynamic features of their N-terminal domains

“…GJ intercellular channels are formed through the assembly of twelve integral membrane proteins called connexins. Six connexins oligomerize to form a hemichannel (also known as connexon) and two hemichannels from neighboring cells can dock together to form a functional gap junction channel if they are docking compatible (White et al, 1994b;Bai et al, 2018).…”

“…Humans express 21 connexin isoforms in a cell-type specific fashion, possibly reflecting the need for unique channel functions that match the physiological demands of their environment. Adding to this diversity, different tissues often express more than one type of connexin, which may coassemble into so-called heteromeric channels (mixed within the same hemichannel) and/or heterotypic channels (mixed between opposing hemichannels) (Koval et al, 2014;Bai et al, 2018). In this way, cells may fine-tune their GJ channel properties to control the synchronization of physiological activities or to maintain homeostasis.…”