TI - Results ESAT-6 caused activation of extracellular signal regulated kinase1/2 ( ERK1/2 ) in cytoplasm but not in nucleus . AB - We studied the effect of ESAT-6 on the activation status of ERK1/2 group of MAP kinases . MAP kinases are activated by a variety of extracellular stimuli such as stress , growth factors and cytokines . The activation of ERK1/2 occurs through phosphorylaTION ; the activated or phosphorylaTED ERK1/2 ( pERK1/2 ) translocate to the nucleus [37] where they PHOSphorylate and activate the downstream cognate transcription factors such as CREB etc. [38] . We found that ESAT-6 ( 5 mug/ml ) caused a time -dependent PHOSphorylation of ERK1/2 (Fig 1A) in cytoplasm of RAW264.7 cells compared to unstimulated cells . In the Figure 1 the ERK1/2 is shown as a doublet where upper band represents ERK-1 with molecular weight of 44 kDa and the lower band represents ERK-2 with molecular weight of 42 kDa . In general cytoplasmic pERK1/2 would have translocated to the nucleus to activate the downstream molecules , but in the case of ESAT-6 -stimulated cells we did not observe any pERK1/2 in the nuclear extract at any of the time points under the observation period ( Fig 1C ) . To determine whether the effect of ESAT-6 was specific for ERK1/2 or not , we checked for the PHOSphorylation of another MAP kinase p38 . ESAT-6 triggered PHOSphorylation of p38 in both cytoplasm ( Fig 1E ) and the nucleus ( Fig 1G ) , therefore the effect of ESAT-6 was specific for ERK1/2 . Total p38 levels were constant over the experimental time period in both cytoplasm ( Fig 1F ) and nucleus ( Fig 1H ) .