Surviving the passage: Non-canonical stromal targeting of an Arabidopsis mitogen-activated protein kinase kinase.

In plants, mitogen-activated protein kinases (MAPK) have been implicated in signalling associated with many processes, including cellular differentiation, organ development, cell death and stress/hormone signalling. While MAPK cascades are known to act in the cytosol and the nucleus, sequence analysis of the Arabidopsis MAPK cascade proteins predicts the presence of import signals that would target some of them to other organelles. In vitro uptake experiments confirm the predicted import of an oxidant-responsive MAPKK, AtMKK4, into the chloroplast. Unexpectedly, the imported MKK4 protein was not processed through stromal peptidase-dependent cleavage of the N-terminal signal peptide, thus leaving the pre-protein intact. Nevertheless, the N-terminal region was shown to be essential both for the import process and for the ability of MKK4 to activate its cognate MAPK targets in vivo. MKK4 import also occurred irrespective of the activation status of the kinase. The import of this primarily cytosolic oxidant-stimulated AtMKK4 into the chloroplasts, organelles with high redox fluxes, suggests that one of the functions of MKK4 might be to help coordinate intercompartment responses to cellular redox imbalances.