page_title: Active zone
section_title: Structure
hierarchical_section_title: Active zone / Structure
caption_attribution_description: English: This is diagram of the proteins found in the active zone. This is not an exhaustive diagram as there are likely more proteins within the active zone and additional protein interactions that have yet to be identified. (Grey lines) protein interaction; (Green) scaffolding proteins; (ELKS) ERC/RAB6-interacting/CAST; (PRA) prenylated Rab acceptor; (Rab) Ras-related in brain; (RIM) Rab interacting molecule; (SNAP) synaptosome associated protein; (VDCC) Voltage-dependent Calcium Channel. Adapted from Mittelstaedt, Alvarez-Baron, and Schoch. RIM proteins and their role in synapse function. Biol. Chem., Vol. 391, 599-606
context_page_description: The active zone or synaptic active zone is a term first used by Couteaux and Pecot-Dechavassinein in 1970 to define the site of neurotransmitter release. Two neurons make near contact through structures called synapses allowing them to communicate with each other. As shown in the adjacent diagram, a synapse consists of the presynaptic bouton of one neuron which stores vesicles containing neurotransmitter, and a second, postsynaptic neuron which bears receptors for the neurotransmitter, together with a gap between the two called the synaptic cleft. When an action potential reaches the presynaptic bouton, the contents of the vesicles are released into the synaptic cleft and the released neurotransmitter travels across the cleft to the postsynaptic neuron and activates the receptors on the postsynaptic membrane.
  The active zone is the region in the presynaptic bouton that mediates neurotransmitter release and is composed of the presynaptic membrane and a dense collection of proteins called the cytomatrix at the active zone. The CAZ is seen under the electron microscope to be a dark area close to the membrane.
context_section_description: The active zone is present in all chemical synapses examined so far and is present in all animal species.  The active zones examined so far have at least two features in common, they all have protein dense material that project from the membrane and tethers synaptic vesicles close to the membrane and they have long filamentous projections originating at the membrane and terminating at vesicles slightly farther from the presynaptic membrane.  The protein dense projections vary in size and shape depending on the type of synapse examined.  One striking example of the dense projection is the ribbon synapse (see below) which contains a "ribbon" of protein dense material that is surrounded by a halo of synaptic vesicles and extends perpendicular to the presynaptic membrane and can be as long as 500 nm.  The glutamate synapse contains smaller pyramid like structures that extend about 50 nm from the membrane.  The neuromuscular synapse contains two rows of vesicles with a long proteinaceous band between them that is connected to regularly spaced horizontal ribs extending perpendicular to the band and parallel with the membrane.  These ribs are then connected to the vesicles which are each positioned above a peg in the membrane (presumably a calcium channel). Previous research indicated that the active zone of glutamatergic neurons contained a highly regular array of pyramid shaped protein dense material and  indicated that these pyramids were connected by filaments.  This structure resembled a geometric lattice where vesicles were guided into holes of the lattice.  This attractive model has come into question by recent experiments.  Recent data shows that the glutamatergic active zone does contain the dense protein material projections but these projections were not in a regular array and contained long filaments projecting about 80 nm into the cytoplasm.
  There are at least five major scaffold proteins that are enriched in the active zone; UNC13B/Munc13, RIMS1 (Rab3-interacting molecule), Bassoon, Piccolo/aczonin, ELKS, and liprins-α.  These scaffold proteins are thought to be the constituents of the dense pyramid like structures of the active zone and are thought to bring the synaptic vesicles into close proximity to the presynaptic membrane and the calcium channels.  The protein ELKS binds to the cell adhesion protein, β-neurexin, and other proteins within the complex such as Piccolo and Bassoon. β-neurexin then binds to cell adhesion molecule, neuroligin located on the postsynaptic membrane.  Neuroligin then interacts with proteins that bind to postsynaptic receptors.  Protein interactions like that seen between Piccolo/ELKS/β-neurexin/neuroligin ensures that machinery that mediates vesicle fusion is in close proximity to calcium channels and that vesicle fusion is adjacent to postsynaptic receptors.  This close proximity vesicle fusion and postsynaptic receptors ensures that there is little delay between the activation of the postsynaptic receptors and the release of neurotransmitters.
caption: A diagram of the proteins found in the active zone
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