4.2.7  Regulation of carrier proteins

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(a)  Effects of nutrition on Vmax and Km

Molecular biology is improving our understanding of ion transport as genes encoding membrane proteins are cloned. Transport of K+ is better understood at a molecular level than that of other plant nutrients. Additionally, a mycorrhizal ortho-phosphate transporter is described in Section 3.5. The greatest challenges lie in elucidating ways in which ion carriers are regulated (Glass 1983; Michelet et al. 1994). The following account of the plasticity of uptake systems in plants demon-strates that ion transporter activity is tightly controlled within membranes.

Absorption of a particular ion is affected by the amount of that ionic species already present in the tissue — as con-centration increases, influx decreases in a logical feedback. The effects are specific. For example, roots which are deficient in sulphur absorb sulphate ions about 20 times faster than non-deficient roots. However, the rate of orthophosphate absorption is not increased in sulphur-deficient tissue (Hawkesford and Belcher 1991). Slower rates of absorption when luxury amounts of an ion are present are due to repression of ‘high-affinity’ transporters. Even though ‘high-affinity’ transporters are ‘repressed’ when nutrient requirements are low, ‘low-affinity’ transporters are ‘constitutive’, that is, present and functioning regardless of the amount of the ion in the tissue. However, repression or activation of ‘high-affinity’ transporters is not the sole basis of regulation of ion uptake. In a study of ‘high-affinity’ K+ absorption by ryegrass and white clover roots, Km and Vmax changed with K+ status of tissues (Figure 4.17); higher internal K+ concentrations reduced the affinity of carrier proteins for K+ and slowed the maximum rate of K+ absorption. In addition to the site directly involved in transporting ions, transporters must have other sites which bind substrate ions. These other sites are situated on a cytoplasmic domain of the protein and influence ion transport, but only in an indirect ‘allosteric’ way. When there is an increase in the cytoplasmic concentration of the ion, a higher proportion of these binding sites is occupied by ions. A process-based model of ion transport predicts four binding sites for this ‘high-affinity’ K+ transporter (Glass 1976). Binding causes a subtle change in tertiary structure of the transporter, lowering its affinity for transporting ions and reducing the rate of transport. In this way, evolution of the protein has given the plant control over its internal ion status. The details of this allosteric binding are still not defined.

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Figure 4.17 Potassium (K+) absorption was measured as uptake of radioactive Rb+ (a tracer) by roots of plants which had been grown in different levels of K+. In ryegrass, K+ deficiency elevated Vmax without much effect on Km whereas in clover, K+ deficiency elevated affinity for K+ (lower Km) without much effect on maximum uptake rate (Vmax)

Orthophosphate uptake by Arabidopsis seedlings is regulated differently. In this case, Km of the ‘high-affinity’ transporter does not change but the rate of absorption increases three-fold when plants are phosphorus deficient. For the ‘low-affinity’ mechanism, Km decreases during phosphorus deficiency (i.e. affinity of orthophosphate for the transporter increases) but, rather unexpectedly, Vmax decreases. Such a response is counter-intuitive but lowering phosphate uptake capacity in plants that are phosphorus deficient might be related to lower growth rates leading to lower nutrient demand. For orthophosphate ions, an allosteric regulation of absorption would not be expected to operate in the same way as it does for K+. The cytoplasmic concentration of orthophosphate ions is tightly controlled and it only changes when phosphorus deficiency is very severe. Low cytoplasmic concentrations cannot therefore provide the basis for an allosteric feedback on uptake. In the case of orthophosphate, and possibly other ions, there is another factor contributing to the regulation of net absorption — an efflux of orthophosphate from the cells. This efflux is significantly lower in plants that are phosphorus deficient (see Case study 4.1).

(b)  Specificity of ion carrier proteins

Ion transporters can show very high levels of specificity. Ions which are very similar chemically to the transported ion may not be transported at all. For example, the ‘high-affinity’ K+ transporter recognises K+ but not Na+ ions even though the two are chemically very similar (they are adjacent alkali metals in the Periodic Table). However, the same transporter absorbs Rb+ with very similar kinetics to K+ absorption; Rb+ is also an alkali metal but is larger than K+. When K+ and Rb+ ions are both present, they compete for the transporter according to their relative concentrations (see Figure 1 in Case study 4.1). Transport appears to require binding of ions to a recognition site on the transporter, conferring specificity. Other examples of specificities are transporters which absorb Zn2+ and Cu2+ but not Ca2+, Mg2+, Mn2+, Fe2+ or Co2+ and one which trans-ports Cl and Br but not F or I. The ions NO3 and ClO3 are also absorbed by the same transporter. In some experi-ments, good use has been made of these specificities. The radioactive isotope of rubidium, 86Rb+, is widely used as a tracer for K+ because the K+ isotope has an inconveniently short half-life. Similarly 36ClO3 is used as tracer for NO3 for which there is no useful radioactive form.

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