DENOVO SYNTHESIS OF PLASMA-MEMBRANE AND TONOPLAST POLYPEPTIDES OF BARLEY ROOTS DURING SHORT-TERM K+ DEPRIVATION - IN SEARCH OF THE HIGH-AFFINITY K+ TRANSPORT-SYSTEM

[S-35]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 S-35 incorporation into membranes when [S-35]methionine was administered for 6 h following 6 h of K+ deprivation. With increasing duration of K+ deprivation, S-35 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+(Rb-86) 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.