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Environmental and Stress Physiology

De Novo Synthesis of Plasma Membrane and Tonoplast Polypeptides of Barley Roots during Short-Term K⁺ Deprivation ¹

In Search of the High-Affinity K⁺ Transport System

Department of Botany, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4

[³⁵S]Methionine labeling of intact barley roots (Hordeum vulgare cv Klondike) after short (6-12 h) and longer (18-24 and 90-96 h) periods of K⁺ deprivation revealed that several membrane polypeptides were synthesized in significantly increased amounts following withdrawal of K⁺ from nutrient solutions. One of these, a 43-kD polypeptide localized in plasma membrane- and tonoplast-enriched fractions, accounted for a large part of ³⁵S incorporation into membranes when [³⁵S]methionine was administered for 6 h following 6 h of K⁺ deprivation. With increasing duration of K⁺ deprivation, ³⁵S incorporation into this 43-kD polypeptide decreased. This polypeptide, referred to as KR43, was not synthesized when NO₃^– or inorganic phosphate was removed or when Rb⁺ was substituted for K⁺. However, it was synthesized when K⁺ was removed and replaced by an equivalent concentration of Na⁺. The intrinsic nature of this polypeptide and the time course of changes in its expression, which correspond with changes of K⁺(⁸⁶Rb) influx associated with K⁺ deprivation, provide evidence that this polypeptide may form part of the high-affinity K⁺ transport system in barley roots. A possible role for this polypeptide is discussed in the context of changes in the subcellular distribution of K⁺ in barley roots following interruption of K⁺ supply. A 45-kD microsomal polypeptide, identified in earlier studies as a response to K⁺ deprivation, is suggested to be an extrinsic protein, readily displaced from membranes by exposure to ethylenediaminetetraacetate.

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