Bifunctional Proteins Respond to Cold Stress

Gene expression in plants is altered in response to cold, and this change contributes to the ability of plants to tolerate cold temperatures. Lee et al. used a luminescent reporter gene screen to identify a mutant ( los2 ) that did not activate the stress-responsive reporter gene when the plants were subjected to cold. The los2 defect was specific to the cold response, but not other stresses, and los2 plants were unable to acclimatize to cold. The los2 mutant plants also exhibited a wilting phenotype when exposed to light and cold. The LOS2 gene encodes enolase (an enzyme in the glycolytic pathway). The enzymatic activity of los2 mutant plants was reduced compared with the activity in wild-type plants, as was the activity of recombinant mutant Los2 protein in vitro compared with that of wild-type recombinant LOS2. LOS2 was located in the cytoplasm and nucleus, as revealed by fluorescent monitoring of a green fluorescent protein fusion. Like a human enolase (also known as MBP1), which is both an enolase and transcriptional regulator, LOS2 also appears to be a transcription factor. LOS2 bound to the c- myc promoter and a more physiological plant target, the STZ/ZAT10 promoter, in electrophoretic mobility-shift assays. STZ/ZAT10 is a transcriptional repressor that can inhibit expression of genes from the cold responsive promoter, and its expression is transiently increased by cold treatment. In the los2 mutant, cold stimulates a prolonged increase in STZ/ZAT10 mRNA. Thus, LOS2 is an enolase and appears to be a transcriptional repressor for STZ/ZAT10, which accounts for its role in cold responses. H. Lee, Y. Guo, M. Ohta, L. Xiong, B. Stevenson, J.-K. Zhu, LOS2 , a genetic locus required for cold-responsive gene transcription encodes a bi-functional enolase. EMBO J. 21 , 2692-2702 (2002). [Abstract] [Full Text]