The KdpFABC complex – K⁺transport against all odds

Abstract

In bacteria, K þ is used to maintain cell volume and osmotic potential. Homeostasis normally involves a network of constitutively expressed transport systems, but in K þ deficient environments, the KdpFABC complex uses ATP to pump K þ into the cell. This complex appears to be a hybrid of two types of transporters, with KdpA descending from the superfamily of K þ transporters and KdpB belonging to the superfamily of P-type ATPases. Studies of enzymatic activity documented a catalytic cycle with hallmarks of classical P-type ATPases and studies of ion transport indicated that K þ import into the cytosol occurred in the second half of this cycle in conjunction with hydrolysis of an aspartyl phosphate intermediate. Atomic structures of the KdpFABC complex from X-ray crystallography and cryo-EM have recently revealed conformations before and after formation of this aspartyl phosphate that appear to contradict the functional studies. Specifically, structural comparisons with the archetypal P-type ATPase, SERCA, suggest that K þ transport occurs in the first half of the cycle, accompanying formation of the aspartyl phosphate. Further controversy has arisen regarding the path by which K þ crosses the membrane. The X-ray structure supports the conventional view that KdpA provides the conduit, whereas cryo-EM structures suggest that K þ moves from KdpA through a long, intramembrane tunnel to reach canonical ion binding sites in KdpB from which they are released to the cytosol. This review discusses evidence supporting these contradictory models and identifies key experiments needed to resolve discrepancies and produce a unified model for this fascinating mechanistic hybrid.Abbreviations: AMPPNP: Adenylyl-imidodiphosphate; BLM: black lipid membrane; DiSC3(5): 3

Serine phosphorylation regulates the P-type potassium pump KdpFABC

“…Interestingly, the E161Q and S162D mutations also resulted in uncoupling, which suggests that this crucial TGES 162 loop has long-range allosteric effects over K + sensing within the transmembrane domain. Although the path taken by K + across the membrane is still uncertain (Pedersen et al, 2019), the canonical substrate binding site on M4 is connected directly to the P-domain and is likely to play an important role in sensing K + and initiating EP formation. In addition, work on other P-type ATPases has shown how large movements of the A-domain propagate to the transmembrane domain via its linkages to M1 and M2 helices.…”

Structural basis for Potassium transport by KdpFABC

“…1b). However, mechanisms for energy coupling between these two subunits have remained elusive, as has the specific transport pathway of K + through the complex (Pedersen et al, 2019).…”

Partial Reactions of the Na,K-ATPase: Determination of Activation Energies and an Approach to Mechanism

“…In contrast, in case of P-type ATPases mechanistic concepts are still missing of how the chemical energy provided by ATP hydrolysis is transformed into an electrochemical potential gradient. While the proposed pathways for the transported ions are rather convincing for the SR Ca-ATPase (Møller et al 2010 ), Na,K-ATPase (Apell 2019 ), and the KdpFABC complex (Pedersen et al 2019 ), mainly based on structural investigations with high resolution, a mechanistic concept of energy transduction, however, could not be advanced beyond speculation so far.…”