Programme Contents

A current source and a cation conductance are components of an electrical circuit connected across the plasma membrane of the malaria parasite Plasmodium falciparum

Like most cells, the intraerythrocytic malaria parasite Plasmodium falciparum requires a high intracellular concentration of K⁺ (∼135 mM) for normal development. Using ⁸⁶Rb⁺ and the potential-sensitive compound ³H-TPP⁺, we have shown that the parasite’s mechanism of K⁺ uptake is electrophoretic, mediated by a pathway with characteristics of a K⁺ channel. The driving force, the parasite’s membrane potential, Δψ, originates from the extrusion of H⁺ by a (V-type) H⁺-ATPase on the plasma membrane. However, we have also shown that Δψ is modulated (partially offset) by extracellular K⁺, indicating an interdependence between K⁺ influx and Δψ.

Investigations into the kinetics of K⁺ uptake have shown that between 5 mM – 130 mM K⁺, the influx of K⁺ remains constant, despite there being a reduction in Δψ with increasing concentrations of extracellular K⁺.

These phenomena may be reconciled by considering the H⁺-ATPase as an ‘ideal’ current source, and the K⁺ channel as a ‘variable’ conductance, the latter a function of the extracellular concentration of K⁺ (see figure). In this electrical model, the inward current carried by K⁺ influx through the K⁺ channel, ‘I_in’, is equal to the outward current carried by the (net) export of H⁺ via the H⁺-ATPase, ‘I_out’ (i.e. I_in = I_out). As the K⁺ conductance of the membrane is varied by altering the extracellular concentration of K⁺, the offset to Δψ caused by the influx of K⁺ also varies, so that the equality I_in = I_out remains satisfied.

During its growth phase, the accumulation of K⁺ by the parasite is achieved in the context of a >10-fold decrease in the concentration of K⁺ (from ∼140 mM) within the host red cell (itself a result of the parasite manipulating the permeability of the host cell membrane). The mechanism we describe is able to explain the parasite’s ability to generate a stable influx of K⁺, neither overwhelmed by, nor starved of, K⁺, as the concentration of K⁺ within the red cell undergoes a dramatic reduction.

Largely on the basis of sequence homology to the canonical selectivity filter of homotetrameric K⁺ channels, two putative K⁺ channel genes have been identified in the Plasmodium falciparum genome database. Hydropathy profiles suggest that both channels have additional transmembrane domains over and above the 6 characteristic of voltage-gated K⁺ channels, a feature shared by several members of the ‘slo’ K⁺ channel family. The function of these domains is unknown. Both channels are unusual for their great size (the larger has ∼2000 residues per subunit), and have large hydrophilic domains which are predicted to reside cytosolically, the functions of which are also unknown. The larger protein has an ‘S4’ segment containing 3 regularly spaced arginines, in a pattern consistent with a (perhaps degenerate?) voltage sensor of a voltage-gated K⁺ channel. Immunofluorescence studies demonstrate localisation of this protein to be predominantly at the host cell membrane, suggesting that it is not the K⁺ uptake pathway in the parasite membrane discussed above, but perhaps plays a role in the alteration of the ionic makeup of the host cell cytosol by the parasite. No data currently exists for the location of the smaller protein. These putative K⁺ channels are the subject of continuing investigations.