JXB Advance Access originally published online on January 13, 2007
Journal of Experimental Botany 2007 58(5):1001-1011; doi:10.1093/jxb/erl260
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© 2007 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details)
RESEARCH PAPER |
Cell division in the unicellular microalga Dunaliella viridis depends on phosphorylation of extracellular signal-regulated kinases (ERKs)
1Department of Ecology, Faculty of Sciences, University of Málaga, E-29071 Málaga, Spain
2Division of Renal Diseases and Hypertension, School of Medicine, University of Colorado Health Sciences Center, Denver, CO 80262, USA
* To whom correspondence should be addressed. E-mail: juan.capasso{at}uchsc.edu
In mammalian cells, MAPKs are involved in both stress response (JNK and p38 pathways) and cell proliferation and differentiation [extracellular signal-regulated kinase (ERK)] through protein kinase cascades. Exposure of Dunaliella viridis cell cultures to PD98059, a very specific inhibitor of the ERK signalling pathway, resulted in a total arrest of cell proliferation and a complete dephosphorylation of ERK. As shown by flow cytometry analysis of propidium iodide-stained cells, PD98059 stopped mitosis at the G2 phase after the S phase has been completed. Multiple physiological parameters such as cell motility and reducing power generation (NADPH) clearly indicate that the treated cells are wholly viable. Exposure of D. viridis to environmental stresses that impair cell division, such as hyperosmotic shock, nitrogen starvation, or sublethal UV irradiation, caused a marked decrease in the phospho-ERK levels as detected by western blot. Two 400 bp polynucleotides from D. viridis with high homologies to published sequences of ERK1 and ERK2 were cloned, sequenced, and submitted to GenBank. Northern blot analysis revealed two mRNA bands of 
1.9 kb, consistent with the expected size of ERK proteins (40 kDa). Sequence analysis showed that they contained several mitogen-activated protein kinase (MAPK) conserved domains, including II, III, VIb, VII, and the double phosphorylation motif. Interestingly, in D. viridis, this motif was T*DY* instead of the canonic T*EY*. Based on this finding, ERK plant sequences can be divided into two groups, one termed the T*DY* branch and the other termed the T*EY* branch. The molecular and functional data presented here suggest that ERK is a very ancient signalling pathway and that it was already present in the last common ancestor of all eukaryotic cells.
Key words: Cell division, Dunaliella, ERK, environmental stress
Received 17 July 2006; Revised 30 October 2006 Accepted 1 November 2006