Target Synonyms and Classification: The K+ uptake ATPase (Kdp ATPase) belongs to the p-type ATPase Superfamily (TCDB: 3.A.3), which catalyze cation transport driven by ATP hydrolysis. It is rather a unique member of this superfamily, since it contains four subunits (KdpFABC).
Function and Mechanism: Kdp ATPase is an inducible high-affinity K+ transporter that is synthesized under conditions of severe K+ limitation or osmotic upshift. It maintains the desired concentration of internal K+ required for various physiological processes like turgor homeostasis and pH regulation. Kdp is composed of four protein subunits forming the KdpFABC complex. Here KdpA acts as a K+ transporter with high affinity (KD = 2 µM). It can maintain cytoplasmic potassium concentrations against gradients up to 10.000 fold. KdpB contains the ATPase domain and KdpC and KdpF may facilitate complex assembly and stabilize the complex. There is a very special mechanism in place which guarantees coupling between KdpA and KdpB: KdpA has a protein embedded tunnel and the cytoplasmic gate within KdpA is linked to the phosphorylation domain of KdpB. Here, a channel architecture was repurposed for active transport. Furthermore, KdpFABC acts as a functional dimer.
Organism and Localization: The Kdp system is widely dispersed among the different classes of bacteria including the cyanobacteria and was studied most in Escherichia coli. It localizes to the plasma membrane. Kdp activity is essential for cell growth at low potassium concentrations.
Substrates and Inhibitors: KdpA has a very high specificity for potassium: Rb+, with a Pauling radius (1.49 Å) comparable to that of K+ (1.33 Å) is transported with much lower affinity (KM = 8 mM vs. KM = 2 µM). It was shown that KdpA depends on pH, showing highest K+ turnover rates at pH 7.4 – likely due to a proton leak pathway. The G232D mutant of KdpA instead transports various monovalent cations (K+ > Rb+ > Na+ > Li+ > H+). A common inhibitor for inhibition of p-type ATPase is orthovanadate. It inhibits the Kdp complex at 1 – 10 µM by trapping the protein in a transition state of phosphoenzyme hydrolysis.
Related Transporters: While KdpB is homologues to p-type ATPase a-subunits, KdpA is homologous to other K+ transporters like KcsA and KtrB. It contains a selectivity filter descenced from that of the bacterial channel KcsA.
Target Synonyms and Classification: The high affinity choline transport 1 (CHT1) belongs to the Solute:Sodium Symporter Family (SSSF, TCDB: 2.A.21), which catalyze the cotransport of sodium and sugars, amino acids or other organo cations. Since it is a human transporter, it’s also classified as an SLC transporter: CHT1 equals to SLC5a7 and belongs to the sodium/glucose cotransport family (SLC5).
Function and Mechanism: CHT1 cotransports Na+, Cl- and choline+ in variable stoichiometry, depending on membrane voltage: 10 charges are translocated at – 80 mV; 3 at -20 mV. Leak currents in absence of choline have been described; CHT1 is classified as a high-affinity / low-capacity transporter with apparent affinity for choline in the range of KM = 2 µM and a maximum turnover rate of 14 molecules per second under physiological conditions; CHT1 has apparent affinities of KM(Na+) = 80 mM and KM(Cl-) = 50 mM. Internal sodium, but not internal chloride reduces choline induced transport, which lead to the conclusion that sodium release is the rate limiting step in the transport cycle. External chloride is required for choline transport, but Cl- may not be cotransported and rather plays a regulatory role. CHT1 also depends on external pH: CHT1 shows highest Vmax at pH 9.5, at pH 5.5 transport by CHT1 is completely abolished. Within this pH interval, the CHT1 transport activity follows a titration curve with pK = 7.4. Besides affecting Vmax, KM for Na+ decreases from 67 mM (pH 6.5) to 13 mM (pH 8.5) at – 60 mV.
Organism and Localization: CHT1 is mainly expressed in presynaptic cholinergic nerve terminals. Its predominant localization is in intracellular organelles, such as endosomal compartments and synaptic vesicles. CHT1 is translocated to the plasma membrane in response to neuronal activity and is then rapidly internalized by endocytosis. CHT1 actively transports extracellular choline into the presynaptic terminals, where it is used for synthesis of acetyl choline. The choline uptake is the rate limiting step in neuronal acetyl choline synthesis; mutations in CHT1 affecting surface transporter trafficking have been found to lead to hereditary motor neuropathy. Abnormal regulation of CHT contributes to cognitive imparimants and neuropsychiatric disorders, including Alzheimer’s disease.
Substrates and Inhibitors: CHT1 activity can be blocked by very low concentrations of hemicholinium-3 (HC-3) with an IC50 of 1 – 5 nM. Hence, CHT1 is also known as the Hemicholinium-3-sensitive choline transporter.
Related Transporters: There are two other mammalian families of choline transporters: The ubiquitously expressed, poly-specific, low-affinity organic cation transporters OCT1 and OCT2 are uniporters, belong to the SLC22 family and Major Facilitator Superfamily (TCDB: 2.A.1). The choline transporter-like (CTL) proteins (TCDB: 2.A.92) have intermediate affinity for choline and are expressed in the central nervous system. The 220 known SLC5 members – to which CHT1 belongs – have diverse function: These transporters transport glucose, myo-inositol and iodide, one is an anion transporter, and another is a glucose-activated ion channel. Members of this family also behave as uniporters, urea and water channels, and urea and water cotransporters.