Several studies focusing on elucidating the mechanism of NO (nitric oxide) signalling in plant cells have highlighted that its biological effects are partly mediated by protein kinases. The identity of these kinases and details of how NO modulates their activities, however, remain poorly investigated. In the present study, we have attempted to clarify the mechanisms underlying NO action in the regulation of NtOSAK (Nicotiana tabacum osmotic stress-activated protein kinase), a member of the SNF1 (sucrose non-fermenting 1)-related protein kinase 2 family. We found that in tobacco BY-2 (bright-yellow 2) cells exposed to salt stress, NtOSAK is rapidly activated, partly through a NO-dependent process. This activation, as well as the one observed following treatment of BY-2 cells with the NO donor DEA/NO (diethylamine-NONOate), involved the phosphorylation of two residues located in the kinase activation loop, one being identified as Ser158. Our results indicate that NtOSAK does not undergo the direct chemical modifications of its cysteine residues by S-nitrosylation. Using a co-immunoprecipitation-based strategy, we identified several proteins present in immunocomplex with NtOSAK in salt-treated cells including the glycolytic enzyme GAPDH (glyceraldehyde-3-phosphate dehydrogenase). Our results indicate that NtOSAK directly interacts with GAPDH in planta. Furthermore, in response to salt, GAPDH showed a transient increase in its S-nitrosylation level which was correlated with the time course of NtOSAK activation. However, GADPH S-nitrosylation did not influence its interaction with NtOSAK and did not have an impact on the activity of the protein kinase. Taken together, the results support the hypothesis that NtOSAK and GAPDH form a cellular complex and that both proteins are regulated directly or indirectly by NO.

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