4.2.6  Transport proteins underlying the dual mechanism of potassium uptake

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Figure 4.16   Dual isotherm of potassium influx into barley roots showing high- and low-affinity uptake systems operating at low and high (K+) concentration ranges, respectively (Based on Epstein 1976)

In many cases, more than one membrane transport mechanism contributes to active influx of a given ion. When roots absorb ions, there appear to be at least two influx mechanisms for each species of ion; K+ is a particularly well studied example (Epstein 1976). The principal evidence for a dual mechanism comes from the relationship between K+ concentration and the rate of K+ absorption (i.e. kinetics of K+ absorption). In practice this is done by measuring how much of a radioactive tracer for K+ is absorbed in a standard length of time from solutions at a range of concentrations. Plotting absorption against concentration gives a curve which does not fit the mathematical function predicted for a single membrane trans-porter. However, proposing that there are two transporters with different affinities for K+ explains the data better. The exact figures depend on the plant species studied and experimental conditions but typically the ‘high-affinity’ phase of uptake (Mechanism I) operates at half its maximum rate (Vmax) at a concentration (Km) of about 0.02 mM whereas Km for the ‘low-affinity’ phase (Mechanism II) is around 1000 times higher at 20 mM (Figure 4.16). The ‘high-affinity’ phase operates at external concentrations below about 0.5 mM; above this concentration, ‘low-affinity’ uptake dominates influx.

In addition to the difference in Km, there are other differ-ences between the two uptake systems. ‘High-affinity’ K+ uptake is insensitive to (1) presence of Na+ and (2) which counter-ion accompanies K+ (e.g. Cl, SO42–). These ions can have a marked effect on ‘low-affinity’ uptake. Calcium ions can stimulate ‘high-affinity’ but they inhibit ‘low-affinity’ uptake. The membrane proteins which give rise to these uptake phases are now being identified by molecular cloning and patch clamping. The following is an account of the proteins which are thought to catalyse ‘high-affinity’ and ‘low-affinity’ K+ transport.

A ‘high-affinity’ K+ transporter from wheat roots has been cloned (reviewed by Maathuis and Sanders 1996) and the mem-brane protein involved named HKT1. Patch clamp ex-periments on protoplasts from Arabidopsis thaliana roots confirm that this high-affinity K+ transporter is an H+/K+ symporter. This advance in our understanding was made possible by expressing a gene encoding HKT1 in Xenopus oocytes (Section 4.1.3(a)) and showing that they could absorb Rb+ ions (as a tracer for K+) with similar kinetic properties to roots (a Km of about 0.029 mM and selectivity for K+ over Na+, Rb+, Cs+ and NH4+). Patch clamp experiments were then performed on these transformed oocytes, and roots of Arabidopsis thaliana, to solve a question about the high-affinity K+ transporter which has long been debated — exactly how is it energised? By studying the effects of pH on the current associated with K+ influx it was shown that HKT1 is a symporter which carries one H+ ion with each K+ ion. Energy is derived from the proton gradient set up by H+-ATPase in the plasma membrane pumping H+ ions into the apoplasm (Section 4.1.3(b)). As H+ ions return to the cytoplasm down this energy gradient via the symporter, the energy released drives K+ influx. In wheat, the expression of HKT1 is localised to cortical cells of roots and cells bordering the vascular tissues in leaves. Both cell types are important in absorbing nutrients, from soils in one case and leaf xylem in the other.

The low-affinity K+ transporter is thought to be a K+ channel that only allows influx (an inward-rectifying channel). The low-affinity K+ transporter resembles an inward-rectifying K+ channel in that it has (1) similar responses to K+ concen-tration (Km = 4 mM) and (2) similar ion selectivities. This K+ channel is voltage dependent and only open at membrane potentials more negative than about –100 mV. When the electro-genic H+-ATPase across the plasma membrane is operating, the membrane potential is normally in excess of –150 mV. Somewhat analogous to the orthophosphate channel discussed in Section 4.2.4, the electrical potential not only provides energy for K+ transport but also the condition necessary to open the channel. The gene KAT1, which has been isolated from Arabidopsis, encodes a protein which is an inward-rectifying K+ channel and has these properties (Section 4.1.3(a)). In this way, a channel and a strong gradient in electrical potential are able to sustain high rates of K+ influx.