User:Avaneesh.T

= Cpx Two component System = One of the many Prokaryotic Two-component regulatory system, The Cpx system serves as the sensor for diverse set of signals and in turn regulates a large number of genes. It consists of the inner membrane bound Sensor protein CpxA, an Histidine kinase which has both kinase and phosphatase activity and acts as the sensor, and the the cytosolic Transcriptional Regulatory protein CpxR which acts as the response regulator. CpxA is autophosphorylated and it transfers the phosphate group to CpxR which regulates gene expression by binding to the promoter region of various genes. The autophosphorylation of CpxA is stimulated signals like alkaline pH, altered membrane lipid composition interaction with hydrophobic surfaces and osmolarity, however the exact nature of the CpxA sensing mechanism is not known.

Activating Signals
The identified upstream effectors of the Cpx Two component system are very diverse and very well characterized. However the actual mechanism of activation is not yet understood. The large periplasmic domain of the CpxA protein is important for the sensing of the signal, and it stimulates the dimerization and subsequent auto-phosphorylation of the cytosolic Histidine kinase domain. Various extracellular and membrane proteins regulate the activation of CpxA, many of which are in turn regulated by the same.

Genes and Operons under regulation of Cpx
Phsophorylation of the Asp51 amino acid of CpxR by the CpxA Kinase domain, activates it and increases its affinity to its target DNA sequence. Binding of the phosphorylated CpxR protein to its target DNA sequence regulates the transcription of the downstream genes. Over 50 genes in 34 operons are regulated by this pathway. The sequence is located within 100bp of the transcription start site, however neither the sequence nor the direction is known to influence regulation significantly. Phosphorylated CpxR is also known to be dephosphorylated by the phosphatase activity of CpxA, which attenuates the signal.

Other interactions
Characterization of all two component systems showed that significant cross talk does exist between the sensor histidine kinases of the prokaryotic cells.

However it was seen that This suggests that Cpx might be a master system in the two component hierarchy.
 * No other two component response regulator other than CpxR is able to cause CpxA dephosphorylation.
 * Only CpxA is able to phosphorylate CpxR, and no transphosphorylation occurs.
 * CpxR is able to enhance dephosphorylation of 5 other Two component Histidine kinases.

CpxA
A classic prokaryotic two component sensor histidine kinase, CpxA is 457 amino acids long. It has three funtional domains : a periplasmic regulatory domain sandwiched by two trans-membrane helices, and the cytoplasmic HAMP(HAMP linker domain) domain and a C-terminal Kinase domain. It is controlled by autoregulation and feedback inhibition by the negetive regulator CpxP.

CpxR
A prokaryotic cytosolic transcriptional regulator, CpxR is active in its phosphrylated state, binding to DNA sequences upstreme of its target genes like : DsbA, PpiA, PpiD, etc. It is 232 amino acids long, and is phosphorylated at the Asp51 residue. It appears to be a modulator that acts in conjunction with other regulators to positively or negetively control the transcription of target genes. In absence of other regulators CpxR is found to not act as a transcriptional activator. This suggests that its role is to enhance the activation of genes controlled by other signalling pathways.

CpxP
A negative regulator of the CpxA Histidine kinase, CpxP is thought to indicate misfolding of envelop proteins. CpxP is found to decrease the auto-phosphorylation of CpxA but not the Phosphotransfer or the Phosphatase activity of the same.