Structural Biochemistry/Membrane Proteins/Gap Junctions

Gap Junctions
/Gap junctions/Gap junctions The gap junctions are strictly packed into a pattern of hexagonal lattice. With the aid of an electron micrograph, each gap junction can be observed to have a 20 Armstrong central hole, which is the lumen of the membrane channel. The channels are also condensed in localized regions of the cell plasma membrane of contiguous cells, and they are spread out in the gap between neighboring cells. The distance of the gap between the cytoplasm of the two apposed cells is roughly 35 Armstrong.

A typical gap junction is composed of connexin, which is a transmembrane protein that varies in mass from 30 to 42 kd. One connexin possesses four membrane-spanning helices. Connexon (hemichannel) are half channels made up of six connexin molecules that are shaped into an extensive hexagonally lattice. A pair of connexons may connect end to end in the gap between adjacent cells to generate a special channel between neighbor cells in order to communicate. Our genome contains twenty-one different connexins. Different tissues un-codes the various connexins in distinguish ways.



Picture form Wikipedia.

In three significant ways, gap junctions are unique compared to membrane channels:

1. The cell-to-cell channels cross two membranes versus just one membrane.

2. The channels join cytoplasm to cytoplasm, and not to the extracellular gap or the lumen of any organelle.

3. Different cells produced the connexons forming the cell-to-cell channels.

The gap junctions tend to remain open for minutes or seconds after they are made because the channels are quickly formed when to cells are in contact. The junctions will be closed if exposed to acidic conditions or high levels of calcium ion. The gap junction closure by calcium ions and protons is a self-saving mechanism of the cell to protect/seal normal cells from injured or dying cells near by. Membrane potential and hormone- assisted phosphorlyation help maintain the gap junctions.

Again, gap junctions are passageway for cell communication. Small ions and hydrophilic substances can flow through the cell-to-cell channel. The pore size of the channel can be experimentally deduced by microinjecting several fluorescent particle into the observed cells and tracking the fluorescent molecule movement to the adjacent cells. The gap junctions facilitated the passage of inorganic ions, amino acids, sugars, and nucleotides in the insides of cells. Large molecules, such as proteins, polysaccharide, and nucleic acid cannot flow through the gap junctions because of their hefty size. Overall, most polar particles that are less than 1kd may easily pass through the cell junction.

Biological Importance of the Gap Junction

Intercellular nourishment and communication are made possible by the cell-to-cell channels. For instance, the cell-to-cell channel plays a key role in providing nourishment to cells that are further away from the blood vessels; bone and lens cells are among numerous cells that can obtain nourishment via gap junctions rather than directly from the blood vessels. Concerning communication between cells, the channels affect cells in strength tissues, such as the heart muscle, are complemented by the intense rush of ions through the protein channel to signify a quick response to the appropriate trigger or stimuli.

The gap junctions govern some aspects of cellular differentiation and growth. The creation of cell-to-cell channels during labor (delivery of a new born) the uterus no longer shelters the baby but is forced to push the baby out by multiple contractions. This is a classic example of muscle cells acting simultaneously to create contractions because of the formation of gap junctions during delivery.

Since various members of the 21 human connexins are expressed differently in the tissues, a mutation in one of the connexins has major consequences. Take connexin 26 for instance; connexin 26 encodes for vital ear tissues and a mutation in connexin will lead to being deaf (hereditary). This is due to the fact that unsuccessful transportation of secondary messenger molecules (inositol trisphoate) or small ions in between sensory cells for the ears creates deafness.

Reference: Biochemistry. 6th ed. New York : W. H. Freeman and Company, 2007. 373-374. Print.