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A gap junction is a specialized cell junction that directly connects the cytoplasm of two cells.
Describe the functions of gap junctions
Gap junctions allow various molecules and ions to pass freely between cells.
A gap junction channel is composed of two connexons, also known as hemichannels that line up across the intercellular space.
Most gap junction hemichannels are composed of a complex of six connexinproteins, each characterized by four transmembrane domains. Six connexin sub-units assemble to create one connexon, or hemichannel.
Channel composition influences the function of the gap junction.
Gap junctions allow for electrical communication between cells, and also allow the passage of small second messengers.
Gap junctions are expressed in virtually all tissues and cells, but most notably in cell types that are involved in direct electrical communication, such as neurons and cardiac muscle.
Gap junctions are also called
communicating junctions, macula communicans, or nexuses. These are connections that allow for the
direct passage of molecules between two cells.
Gap junctions consist of a number of
transmembrane channels called pores that are found in a closely packed arrangement. The number of gap junctions shared between
two cells can vary as well.
Each gap junction channel is
made up of two half channels (hemichannels), one in each cell’s membrane. These half channels join together, bridge the
extracellular space in the process, and form the entire channel that spans both
Each of these half
channels is called a connexon. Each
connexon is made up of six symmetrical integral membrane protein units called
connexins. This means each channel is
made up of 12 circularly arranged protein units.
The molecules that may cross
this channel include the likes of ions, regulatory proteins, and metabolites
(products of metabolism). Examples of
this includes calcium ions and cAMP (cyclic adenosine monophosphate).
Depending on the type of gap junction in
question, molecules can pass evenly in both directions, or asymmetrically, so in some gap junctions the molecules
will move in one direction faster than in the other direction.
The channels in a gap
junction aren’t always open. They
fluctuate between being open and closed.
The ability of the channel to open or close is made possible in
part to calcium ions, which induce a reversible conformational change in the
connexin molecules, which leads to the closure of a channel at its extracellular
surface. The cytoplasmic end of each
connexon can also be closed, if necessary.
Gap junctions are found in
many places throughout the body. This
includes epithelia, which are the coverings of body surfaces, as well as nerves,
cardiac (heart) muscle, and smooth muscle (such as that of the intestines).
Their primary role is to coordinate the
activity of adjacent cells. For
instance, when heart cells need to beat in unison, gap junctions allow for the
transmission of electrical signals between the cells.