User talk:Kanishkatks/kdpFABC Operon

In Escherichia coli, the kdpFABC operon encodes the kdpATPase (comprising the kdpFABC complex), which is an inducible high-affinity Potassium cation (K+)transporter that is synthesized under conditions of severe K+ limitation or osmotic upshift. A typical bacterial two-component signal transduction system constituted by the kdpD and kdpE proteins that are responsible for the transcriptional regulation of the E. coli kdp expression. Various physiological processes like pH regulation, turgor homeostasis, etc. are highly reliant on the K+, which is a dominant intracellular cation. In order to maintain desired concentrations of internal K+, bacterial cells have evolved several different K+ transporters. The kdpFABC Operon of E. coli consists of four genes viz. kdpA, kdpB, kdpC and kdpF.

kdpATPase System
The kdpFABC complex has been classified as a type IA P-type ATPase based on the properties of the ATP-hydrolyzing subunit (kdpB). kdp has a unique oligomeric composition and segregates potassium transport and ATP hydrolysis onto separate subunits: kdpA and kdpB. The kdp system of E. coli is expressed when turgor pressure is low. This expression requires kdpD, a 99-kDa membrane protein, and kdpE, a 25-kDa soluble cytoplasmic protein. The sequences of kdpD and kdpE show they are members of the sensor-effector class of regulatory proteins. The sites of ATP hydrolysis and substrate transport are spatially separated on two different polypeptides, which, in turn, leads to a unique coupling mechanism. The kdpA subunit mediates the translocation of potassium. The kdpC subunit participates in the binding of ATP, thus acting as a catalytic chaperone, which increases the ATP binding affinity of the kdpB subunit via a mechanism typical of nucleotide binding in ABC transporters. kdpB is homologous to the catalytic subunit of other P-type ATPases and shares a common domain architecture as well as key sequence motifs. kdpB is dominated by a pear-shaped cytoplasmic domain that is connected to the membrane by a narrower stalk. In contrast, kdpA has structural and functional similarities to the pore-forming helices of K+ channels the kdpA subunit has been placed next to kdpB along the tubular densities that run parallel to the membrane surface. The unit cell contains a dimer of kdp, which is mediated [citation needed] by a contact between N-domains of kdpB. Along one unit cell axis, these dimers are linked via the tubular densities, which are believed to be kdpA subunits.

kdpFABC Operon Organization


The genes encoding the structural subunits of the kdp-ATPase (i.e. kdpA, kdpB and kdpC) are arranged in an operon while the genes encoding the regulatory kdpD/E proteins (i.e. kdpDE) constitute another operon. The E. coli kdp operon consists of kdpF, kdpA, kdpB, and kdpC, arranged in that order, while the kdpD and kdpE genes, which constitute a separate operon, are situated downstream of kdpC gene, as shown in figure 1.

Expression of kdp Operons
Potassium limitation: The kdpFABC operon is repressed during growth in media of high external K+ concentration. In E. coli, the kdp expression occurs only when the concentration of external K+ falls below 2 mM. Osmotic pressure: In E. coli, for a given [K+], higher kdp expression is correlated with high osmolarity. Though the effect of osmolarity on expression of kdp has been challenged, it remains unequivocal that turgor pressure has key role in its expression.

Regulation: Two-component regulatory system


Transcriptional activation of the kdp operon is mediated by a membrane-bound sensor kinase kdpD, which upon perception of the appropriate stimulus undergoes auto phosphorylation. The phosphoryl group is subsequently transferred to the cytosolic response regulator kdpE, thereby transforming it into a transcriptional activator kdpE~P. This gets bound to a specific DNA sequence, which is upstream of the kdpFABC promoter and induces transcription of the kdp operon. The input domain involved in stimulus perception by the kdpD is formed by the N-terminal domain (NTD), the trans-membrane segments and a cluster of positively charged arginine residues in the C-terminal domain (CTD).

The NTD includes two motifs, which have been hypothesized to play an important role in fine-tuning the E. coli kdpD activity. The kdpD-CTD harbors a conserved Histidine residue (His-673), where phosphorylation occurs. The presence of ATP (or its non-hydrolysable analogs) substantially increases the phospho-kdpE phosphatase activity of the E. coli kdpD.

Homologs of Kdp sysytem in eukaryotes
In many eukaryotic organisms, P-type ATPases comprise only one subunit, whereas the Kdp complex is composed of four subunits. However, multiple polypeptides forming the transport system can occasionally be found in other bacterial P-type ATPases.In strong contrast, homologs of the Kdp system have not yet been found in any of the eukaryotic genomes sequenced so far, thereby indicating that this K+ transport system is unique for prokaryotes.