Proteins in Plants

Much of what we have learned about 14-3-3 in plants has come from studies of interactions with one of the first plant proteins to be identified as a 14-3-3 client, the enzyme nitrate reductase (NR [reviewed in references 29 and 30]), although other intensively studied interactions, such as with H + ATPase [31] and SPS [32], have also greatly aided our understanding. Nitrate reductance is a key enzyme in plant nitrogen metabolism, converting nitrate to nitrite. In the light, this process is coupled to amino acid synthesis, as reduced ferredoxin is required. In the absence of the photosynthetic process, nitrite would accumulate, resulting in cytotoxicity; thus NR is inactivated in the dark through phosphorylation of a conserved seryl residue and the binding of an NR inhibitor protein (together with divalent cations or polyamines). The NR phosphoprotein-binding inhibitor protein was subsequently identified as 14-3-3 [33, 34]. Several different NR kinases have been characterized, including an SNF1-like kinase and two CDPKs. Interestingly, these protein kinases can also be bound by

14-3-3, but the significance of this is not yet clear. Reactivation of NR activity is by dephosphorylation of the regulatory Ser, but exactly how this occurs is also not understood since bound 14-3-3 physically blocks dephosphorylation; it is possible that 14-3-3 is post-translationally modified to change its binding properties, thus allowing phosphatases to act on the binding site. NR is also proteolytically degraded in the dark, and 14-3-3 binding might be a prerequisite for this. Binding specificity of 14-3-3 to NR was studied in barley and depends on isoform expression patterns in the plant and on the amino acid sequence in loop 8 (between helix 8 and 9) of 14-3-3 [27]. The presence of a specific Gly residue in this cation-binding loop confers additional flexibility to the C-terminal, which can alter target binding and also renders the C-terminal more susceptible to proteolysis [26]. The C terminal of the Arabidopsis m isoform may act as an autoinhibitor of binding, possibly by physically blocking access to the amphipathic groove [35]. Interestingly, the C-terminus of barley isoform A is specifically cleaved in vivo during germination [24]. This proteolytic processing may therefore also contribute to binding specificity [25].

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