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Structure class 1: receptors for cyclic nucleotides, IP3, Ryanodine (1.3)
This superfamily includes the following subtypes of receptors:
- Tab. 1
|1.3.1||Cyclic Nucleotide Receptors|
*: IP3 = Inositol triphosphate
Cyclic nucleotide receptors (cGMP and cAMP) are found in retinal photoreceptor cells as well as in olfactory neurons and sperm. Their subunits are similar to those of voltage-gated potassium channels. Six transmembrane (TM) segments and a loop between TM5 and TM6 are postulated here as well. The loop, tooP-loop When four subunits come together, it forms an essential part of the central channel formed by it (see note below).
Also the receptors for IP3 and ryanodine seem to follow this general structure. These two types of receptors embody intracellular calcium channels, which means that they release the "second messenger" calcium from intracellular stores in response to a corresponding signal. Sit IP3Receptors in the membrane of the endoplasmic reticulum of almost all cell types, while ryanodine receptors predominantly occur in the sarcoplasmic reticulum of muscle cells, where they play a key role in the electromechanical coupling.
- After long trials, it was possible in 1998 for the first time to produce and crystallize a potassium channel protein in sufficient quantities. This cleared the way for an X-ray structure analysis, which provided a unique insight into the structure of the canal pore (Doyle et al., 1998). The channel was a simply structured bacterial potassium channel (KcsA), which was made up of the last two TM segments with the P-loop This structural motif can also be found in structural class 1.6.
Doyle, D. A .; Morais Cabral, J .; Pfuetzner, R. A .; Kuo, A .; Gulbis, J. M .; Cohen, S. L .; Chait, B. T .; MacKinnon, R. (1998):The structure of the potassium channel: molecular basis of K + conduction and selectivity. In: Science. 280, 69-77