Are ye’ a cold kinase? Or are ye’ no a cold kinase?

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I’ve been wrestling with a couple of ‘kinase’ papers of recent. They were both published in December 2017 in the journal Developmental Cell.

Firstly there is Zhao et al. with ‘MAP Kinase Cascades Regulate the Cold Response by Modulating ICE1 Protein Stability

…and the other paper is Li et al. with ‘MPK3- and MPK6-Mediated ICE1 Phosphorylation Negatively Regulates ICE1 Stability and Freezing Tolerance in Arabidopsis

Both papers examine the early response of plants to temperature and the involvement of protein kinases – principally the mitogen-activated protein kinases (MAPKs) e.g. the MAP kinase kinase kinases (MAP3Ks; also called MAPKKKs or MEKKs), MAP kinase kinases (MAP2Ks; also called MKKs or MEKs), and MAPKs… can already see how complicated this can get.

When plants adapt to cold there are large changes in the expression of thousands of genes, and its now well established that these changes are mediated by what are known as the CBF genes and they do this by regulating another subset of genes known as the COR genes. Its a bit like a domino effect. Once cold is triggered the dominoes start falling leading to COR gene expression and the plants physiological response to temperature.

Whereas quite a lot is now known about the later stages of the domino line at the molecular level, less is known about the ‘who’ and ‘what’ starts to triggering the domino line. Moving slightly up from the CBFs are the ICEs (ICE1 and ICE2).  ICE1 is a transcription factor that binds to CBF promoters and activates their expression. ICE is therefore seen as a very interesting ‘domino’ and how it is knocked over (..or activated) in this cascade is of great interest.  Several suspects were on the wanted list, including the MAPKs: MPK3, MPK4 and MPK6 and MEKK1 and MKK2, and these are the focus of both these papers.

I won’t go into much more detail. However, here are the highlights from both papers:

Li et al.:

1. Cold activates mitogen-activated protein kinases MPK3 and MPK6

2. MPK3/MPK6 phosphorylate and destabilises ICE1 protein, and

3. MPK3/MPK6 activation attenuates plant freezing tolerance

Zhao et al. :

1. The MKK4/5-MPK3/6 cascade negatively regulates freezing tolerance

2. The MEKK1-MKK2-MPK4 cascade positively regulates freezing tolerance

3. MPK3/6-mediated phosphorylation of ICE1 promotes ICE1 degradation

I think the other important thing is that Zhao et al. show that MPK4 positively regulates the cold response by constitutively suppressing MPK3 and MPK6 activity i.e. MPK4 blocks the ‘destruction’ of ICE1 by MPK3 and MPK6.

Anyway there is also a commentary article in the same issue in Developmental Cell on the background and features of these two papers called ‘MAP kinase Signaling Turns to ICE’

Must check it out. I think it will articulate these results much better than I can…

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