SNARE (Protein)
SNARE complexes ( Abbreviation for: soluble N- ethylmaleimide - sensitive -factor attachment receptor ) are protein complexes in vesicles of eukaryotic cells. The subunits of these complexes are named according to SNARE proteins. SNARE complexes catalyze the merger of biological membranes in the transport of small molecules, for example, at a exocytosis into the synaptic cleft.
Properties
SNARE complexes occur in eukaryotes in all secreting cells. Nerve cells, for example, retain their neurotransmitter synthesized in vesicles ready collected. If the transmitter outside the cell are released, the vesicles fuse with the membrane and has a pore formed by the transmitter reach the molecules to the outside. The fusion and opening of the vesicle is of SNARE and other proteins ( myosin II) controlled.
The transport equation is:
In humans, nine SNARE proteins are known: synaptobrevin -1 and -2, syntaxin -1A and -4, snap, SNAP -23 and -25, and Endobrevin Vesikelfusionsprotein NSF. From three to five of these proteins a dozen different SNARE complexes are formed at least.
Structure
The SNARE proteins form very stable complexes, each consisting of four parallel, in so-called " coiled- coil " arranged α -helices with a 60-70 amino acids comprehensive SNARE motif. The interior of this core complex is formed (perpendicular to Verdrillungsachse ) from 16 layers of hydrophobic amino acids. An exception is the middle " 0 - layer", the three glutamine (Q) and an arginine (R) - is formed - ie amino acids with polar side chains. Depending on their position in the core complex distinction is Qa -, Qb -, Qc - and R- SNAREs, which are highly conserved evolutionarily. It is assumed that a functional SNARE complex must have QabcR the composition. Some of the SNARE proteins are transmembrane proteins that are in both of the membranes to be fused must occur at least one.
Purpose and function
Located on the vesicle (v- SNARE vesicle = synaptosomes receptor -associated protein ) as well as the target organelle (t- SNARE = target synaptosomes receptor -associated protein ), they ensure the correct entry of mergers, because each organelle or vesicle specific SNARE composition has. The fusion takes place at the molecular stimulus (eg, increasing the Ca2 concentration with synaptotagmin as a sensor of protein); in the fusion of water must be displaced from the hydrophilic (water- loving) vesicle, which is energetically highly unfavorable and thus the specificity is used, namely if sufficient energy is released by the protein interaction only with correct v -and t- SNARE combination. In addition to the SNAREs and Rab proteins play a role in the directed fusion of vesicle and organelle.
The performance- border merger then provides the organelle not only the contents of the vesicle but also new membrane lipids.
SNAREs as the target of neurotoxins
Tetanus toxin and botulinum toxin ( Botox) play a role in the blockade of synapses. They cleave SNARE proteins, which vesicle fusion and thus transmitter release is prevented. A tetanus spasm occurs when tetanus toxin blocks inhibitory synapses. There are seven known botulinum toxins, one of them named toxin A. This protein consists of two major parts, the shorter than endopeptidase acts. Hydrolytically cleaves the SNAP-25 protein, which is located in the presynaptic membrane. Botox blocks, for example, excitatory synapses.