Optimizing the solution conditions to solve the structure of the Connexin43 carboxyl terminus attached to the 4(th) transmembrane domain in detergent micelles

Connexins: Synthesis, Post-Translational Modifications, and Trafficking in Health and Disease

Connexins are tetraspan transmembrane proteins that form gap junctions and facilitate direct intercellular communication, a critical feature for the development, function, and homeostasis of tissues and organs. In addition, a growing number of gap junction-independent functions are being ascribed to these proteins. The connexin gene family is under extensive regulation at the transcriptional and post-transcriptional level, and undergoes numerous modifications at the protein level, including phosphorylation, which ultimately affects their trafficking, stability, and function. Here, we summarize these key regulatory events, with emphasis on how these affect connexin multifunctionality in health and disease.

Secondary structural analysis of the carboxyl-terminal domain from different connexin isoforms

The connexin carboxyl-terminal (CxCT) domain plays a role in the trafficking, localization, and turnover of gap junction channels, as well as the level of gap junction intercellular communication via numerous post-translational modifications and protein-protein interactions. As a key player in the regulation of gap junctions, the CT presents itself as a target for manipulation intended to modify function. Specific to intrinsically disordered proteins, identifying residues whose secondary structure can be manipulated will be critical toward unlocking the therapeutic potential of the CxCT domain. To accomplish this goal, we used biophysical methods to characterize CxCT domains attached to their fourth transmembrane domain (TM4). Circular dichroism and nuclear magnetic resonance were complementary in demonstrating the connexin isoforms that form the greatest amount of α-helical structure in their CT domain (Cx45 > Cx43 > Cx32 > Cx50 > Cx37 ≈ Cx40 ≈ Cx26). Studies compared the influence of 2,2,2-trifluoroethanol, pH, phosphorylation, and mutations (Cx32, X-linked Charcot-Marie Tooth disease; Cx26, hearing loss) on the TM4-CxCT structure. While pH modestly influences the CT structure, a major structural change was associated with phosphomimetic substitutions. Since most connexin CT domains are phosphorylated throughout their life cycle, studies of phospho-TM4-CxCT isoforms will be critical toward understanding the role that structure plays in regulating gap junction function.

The cytosolic tail of the tumor marker protein Trop2--a structural switch triggered by phosphorylation

Trop2 is a transmembrane signaling glycoprotein upregulated in stem and carcinoma cells. Proliferation-enhancing signaling involves regulated intramembrane proteolytic release of a short cytoplasmic fragment, which is later engaged in a cytosolic signaling complex. We propose that Trop2 function is modulated by phosphorylation of a specific serine residue within this cytosolic region (Ser303), and by proximity effects exerted on the cytosolic tail by Trop2 dimerization. Structural characterization of both the transmembrane (Trop2TM) and cytosolic regions (Trop2IC) support this hypothesis, and shows that the central region of Trop2IC forms an α-helix. Comparison of NMR structures of non-phosphorylated and phosphorylated forms suggest that phosphorylation of Trop2IC triggers salt bridge reshuffling, resulting in significant conformational changes including ordering of the C-terminal tail. In addition, we demonstrate that the cytosolic regions of two Trop2 subunits can be brought into close proximity via transmembrane part dimerization. Finally, we show that Ser303-phosphorylation significantly affects the structure and accessibility of functionally important regions of the cytosolic tail. These observed structural features of Trop2 at the membrane-cytosol interface could be important for regulation of Trop2 signaling activity.

Functional analysis and regulation of purified connexin hemichannels

Gap-junction channels (GJCs) are aqueous channels that communicate adjacent cells. They are formed by head-to-head association of two hemichannels (HCs), one from each of the adjacent cells. Functional HCs are connexin hexamers composed of one or more connexin isoforms. Deafness is the most frequent sensineural disorder, and mutations of Cx26 are the most common cause of genetic deafness. Cx43 is the most ubiquitous connexin, expressed in many organs, tissues, and cell types, including heart, brain, and kidney. Alterations in its expression and function play important roles in the pathophysiology of very frequent medical problems such as those related to cardiac and brain ischemia. There is extensive information on the relationship between phosphorylation and Cx43 targeting, location, and function from experiments in cells and organs in normal and pathological conditions. However, the molecular mechanisms of Cx43 regulation by phosphorylation are hard to tackle in complex systems. Here, we present the use of purified HCs as a model for functional and structural studies. Cx26 and Cx43 are the only isoforms that have been purified, reconstituted, and subjected to functional and structural analysis. Purified Cx26 and Cx43 HCs have properties compatible with those demonstrated in cells, and present methodologies for the functional analysis of purified HCs reconstituted in liposomes. We show that phosphorylation of serine 368 by PKC produces a partial closure of the Cx43 HCs, changing solute selectivity. We also present evidence that the effect of phosphorylation is highly cooperative, requiring modification of several connexin subunits, and that phosphorylation of serine 368 elicits conformational changes in the purified HCs. The use of purified HCs is starting to provide critical data to understand the regulation of HCs at the molecular level.

¹H, ¹³C, and ¹⁵N backbone resonance assignments of the connexin43 carboxyl terminal domain attached to the 4th transmembrane domain in detergent micelles

Gap junctions are specialized membrane channels that enable coordination of cellular functions and whole-organ responses by facilitating both molecular and electrical communication between neighboring cells. Connexin43 (Cx43) is the most widely expressed and well-studied gap junction protein. In the heart, Cx43 is essential for normal cardiac development and function. Studies using a soluble version of the Cx43 carboxyl-terminal domain (Cx43CT; S255-I382) have established the central role it plays in channel regulation. However, in purifying and characterizing a more 'native-like' construct (Cx43CT attached to the fourth transmembrane domain (TM4-Cx43CT; D196-I382)), we have identified that the TM4-Cx43CT is a better model than the soluble Cx43CT to further investigate the mechanisms governing Cx43 channel regulation. Here, we report the backbone (1)H, (15)N, and (13)C assignments and predicted secondary structure of the TM4-Cx43CT. Assignment of the TM4-Cx43CT is a key step towards a better understanding of the structural basis of Cx43 regulation, which will lead to improved strategies for modulation of junctional communication that has been altered due to disease or ischemic injury.

Effects of phosphorylation on the structure and backbone dynamics of the intrinsically disordered connexin43 C-terminal domain

Phosphorylation of the connexin43 C-terminal (Cx43CT) domain regulates gap junction intercellular communication. However, an understanding of the mechanisms by which phosphorylation exerts its effects is lacking. Here, we test the hypothesis that phosphorylation regulates Cx43 gap junction intercellular communication by mediating structural changes in the C-terminal domain. Circular dichroism and nuclear magnetic resonance were used to characterize the effects of phosphorylation on the secondary structure and backbone dynamics of soluble and membrane-tethered Cx43CT domains. Cx43CT phospho-mimetic isoforms, which have Asp substitutions at specific Ser/Tyr sites, revealed phosphorylation alters the α-helical content of the Cx43CT domain only when attached to the membrane. The changes in secondary structure are due to variations in the conformational preference and backbone flexibility of residues adjacent and distal to the site(s) of modification. In addition to the known direct effects of phosphorylation on molecular partner interactions, the data presented here suggest phosphorylation may also indirectly regulate binding affinity by altering the conformational preference of the Cx43CT domain.

A history of gap junction structure: hexagonal arrays to atomic resolution

Gap junctions are specialized membrane structures that provide an intercellular pathway for the propagation and/or amplification of signaling cascades responsible for impulse propagation, cell growth, and development. Prior to the identification of the proteins that comprise gap junctions, elucidation of channel structure began with initial observations of a hexagonal nexus connecting apposed cellular membranes. Concomitant with technological advancements spanning over 50 years, atomic resolution structures are now available detailing channel architecture and the cytoplasmic domains that have helped to define mechanisms governing the regulation of gap junctions. Highlighted in this review are the seminal structural studies that have led to our current understanding of gap junction biology.

Voltage-dependent conformational changes in connexin channels

Channels formed by connexins display two distinct types of voltage-dependent gating, termed V(j)- or fast-gating and loop- or slow-gating. Recent studies, using metal bridge formation and chemical cross-linking have identified a region within the channel pore that contributes to the formation of the loop-gate permeability barrier. The conformational changes are remarkably large, reducing the channel pore diameter from 15 to 20Å to less than 4Å. Surprisingly, the largest conformational change occurs in the most stable region of the channel pore, the 3(10) or parahelix formed by amino acids in the 42-51 segment. The data provide a set of positional constraints that can be used to model the structure of the loop-gate closed state. Less is known about the conformation of the V(j)-gate closed state. There appear to be two different mechanisms; one in which conformational changes in channel structure are linked to a voltage sensor contained in the N-terminus of Cx26 and Cx32 and a second in which the C-terminus of Cx43 and Cx40 may act either as a gating particle to block the channel pore or alternatively to stabilize the closed state. The later mechanism utilizes the same domains as implicated in effecting pH gating of Cx43 channels. It is unclear if the two V(j)-gating mechanisms are related or if they represent different gating mechanisms that operate separately in different subsets of connexin channels. A model of the V(j)-closed state of Cx26 hemichannel that is based on the X-ray structure of Cx26 and electron crystallographic structures of a Cx26 mutation suggests that the permeability barrier for V(j)-gating is formed exclusively by the N-terminus, but recent information suggests that this conformation may not represent a voltage-closed state. Closed state models are considered from a thermodynamic perspective based on information from the 3.5Å Cx26 crystal structure and molecular dynamics (MD) simulations. The applications of computational and experimental methods to define the path of allosteric molecular transitions that link the open and closed states are discussed. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.