GJD3 Membrane Protein Introduction

Introduction of GJD3

Gap junction delta-3 protein, also known as Cx31.9, is a protein that in humans is encoded by the GJD3 gene, which is a member of the large family of connexins. Connexins are gap junction proteins which are expressed predominantly in mammalian neurons. They are arranged in groups of 6 around a central pore to form a connexon which is a component of the gap junction intercellular channel. The channels formed by connexins protein allow for the cationic molecule exchange between human beta cells, thus plays a role in the regulation of insulin secretion. Connexins associate in groups of 6 and are organized radially around a central pore to form connexons. Each gap junction intercellular channel is formed by the conjunction of 2 connexons. Among the related pathways of GJD3 are vesicle-mediated transport and gap junction trafficking. Gene Ontology annotations related to this gene include ion channel activity and gap junction channel activity.

Function of GJD3 Membrane Protein

Analysis of the Cx31.9 protein sequence reveals many similarities to other connexins. There are four transmembrane domains, conserved spacing of cysteine residues in the extracellular domains, and a number of potential phosphorylation sites in the COOH-terminal domain. Cx31.9 also contains a stretch of 10 proline residues (residues 238–248) in the COOH-terminal domain. This domain may be involved in protein-protein interactions, because proline-rich sequences are known to bind to a number of specific protein interaction modules, such as Src homology 3 domains.

Cx31.9 forms functional gap junctions and is expressed in vascular smooth muscle cells. One unusual feature of the Cx31.9 gene is the probable lack introns in the 5’ -UTR. Connexin-31.9 protein has been reported to be expressed in a range of human tissues, mainly in vascular smooth muscle cells. In transfected HeLa cells both these connexins, human Cx31.9 exhibits some electrophysiological properties, i.e. very low unitary conductance and low sensitivity to transjunctional voltage. It suggests that human Cx31.9 plays no role in AV-nodal impulse delay or conduction elsewhere in the human heart.

Fig.1 Immunofluorescence analyses on HeLaCx31.9 (I, V) and HeLaCx31.9-EGFP (II–IV, VI–VIII) transfectants with anti-Cx31.9 [R15I] (I–IV) and anti-Cx31.9 [A16A]. (Kreuzberg, 2009)

Application of GJD3 Membrane Protein in Literature

1. Kreuzberg M.M., et al. Human connexin31.9, unlike its orthologous protein connexin30.2 in the mouse, is not detectable in the human cardiac conduction system. J Mol Cell Cardiol. 2009, 46(4): 553-9. PubMed ID: 19168070
   
   

   The study demonstrates that unlike its counterpart in the mouse, Cx31.9 protein is not expressed in detectable quantities and is thus unlikely to contribute to the impulse generation and conduction system or the working myocardium of the human heart.

2. Belluardo N., et al. Identification, and functional expression of HCx31.9, a novel gap junction gene. Cell Commun Adhes. 2001, 8(4-6): 173-8. PubMed ID: 12064584
   
   

   Authors in this group have identified a novel human gap junction gene that encodes a protein designated HCx31.9. It is expressed in human cerebral cortex, liver, heart, spleen, lung, and kidney.

3. Nielsen P.A., et al. Molecular cloning, functional expression, and tissue distribution of a novel human gap junction-forming protein, connexin-31.9. Interaction with zona occludens protein-1. J Biol Chem. 2002, 277(41): 38272-83. PubMed ID: 12154091
   
   

   The article indicates that Cx31.9 represents a novel connexin gene that in vivo generates a protein with unique voltage gating properties.

4. Bukauskas F.F., et al. Properties of mouse connexin 30.2 and human connexin 31.9 hemichannels: implications for atrioventricular conduction in the heart. Proc Natl Acad Sci U S A. 2006, 103(25): 9726-31. PubMed ID: 16772377
   
   

   The article reveals that the hemichannels formed of mCx30.2 and hCx31.9 may slow propagation of excitation in the sinoatrial and atrioventricular nodes by shortening the space constant and depolarizing the excitable membrane.

5. White T.W., et al. Virtual cloning, functional expression, and gating analysis of human connexin31.9. Am J Physiol Cell Physiol. 2002, 283(3): C960-70. PubMed ID: 12176752
   
   

   This article reports a novel human connexin Cx31.9 that forms channels with unique functional properties. Cx31.9 channels are gated by cytoplasmic acidification or exposure to halothane like other connexins.

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Reference

1. Kreuzberg M M, et al. (2009). Human connexin31.9, unlike its orthologous protein connexin30.2 in the mouse, is not detectable in the human cardiac conduction system. J Mol Cell Cardiol. 46(4), 553-9.

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