Published 12 September 2005. doi:10.1083/jcb1706iti3
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 170, Number 6, 861-861
eIF2 activated in spots
The eIF2 translation initiation factor travels to a cytoplasmic focus for activation, according to Campbell et al. (page 925). In stressed cells, which turn down translation, the same foci soak up eIF2.
Stressed yeast cells shut down general translation so they can concentrate on making proteins that will help them adapt. One highly regulated protein during this inhibition is eIF2, which is active only in its GTP-bound form. The GDP-to-GTP exchange is done by eIF2B, about half of which the authors now show aggregates in a cytoplasmic blob in yeast.
Under normal conditions, eIF2 shuttled quickly in and out of the foci, presumably getting activated and sent on its way. But when cells were stressed (e.g., by low amino acid levels), eIF2 was less able to escape the foci. This trapping depended on eIF2 phosphorylation. Translation inhibition also depends on eIF2 phosphorylation, which locks eIF2 in an inactive complex with eIF2B, but it is not clear whether eIF2B must be in foci for inhibition to occur.
The concentration of some eIF2B in foci might make translation more efficient or more easily regulated. The group suspects that the foci themselves move around the cell, like a mop for GDP-bound eIF2. Although no one has seen these foci in mammalian cells, they might have been easily missed: only actively translating yeast cells contained them. 
Nicole LeBrasseur
lebrasn{at}rockefeller.edu
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