Gap junction
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A gap junction is a junction between certain animal/plant cell-types that allows different molecules and ions to pass freely between cells. The junction connects the cytoplasm of cells. They are formed by an assembly of connexons. In mammals, connexons are four-pass transmembrane proteins. At least 24 different members of this family are currently known. Generally, the genes coding for gap junctions are classified in one of three groups, based on sequence similarity: A, B and C (for example, GJA1, GJC1). An alternative naming system based on the protein's molecular weight is also popular and both systems are used interchangeably (for example: connexin43, connexin30.3). Six connexons will form gap junctions by polymerising. They need not necessarily be of the same type. If they are, the resulting connexon is called heteromeric, if they are not, it is called homomeric. Gap junctions are particularly important in the cardiac muscle: the signal to contract is passed efficiently through the gap junctions, allowing the heart muscle cells to contract in tandem. However, they are imperative to all cells throughout the body. This is demonstrated by tumor cells that generally lack any gap junctions.
See also: Electrical synapse, ion channel