Abstract
CELL BIOLOGY The recent explosion of research on the interaction of small RNAs with messenger RNAs (mRNAs), which can lead either to cleavage of the double-stranded RNA complex or to translational repression, has intersected with studies of the life cycles of mRNAs, which in some cases spend part of their time in cytoplasmic processing bodies in a translationally dormant state, before degradation or reactivation. Beliakova-Bethell et al. have observed that the protein and RNA components of the yeast retrovirus-like element Ty3 congregate in cytoplasmic foci that also contain nascent virus-like particles. These sites turn out to be a way station for other proteins already shown to reside in processing bodies, among them Dhh1, a helicase that is involved in translational represssion and is required for Ty3 retrotransposition. Hence it appears that the demands of assembling proteins onto an RNA genome may be facilitated by the translational stasis imposed within processing bodies. — GJC RNA 12 , 94 (2006).
Highlights
Two-component systems enable microbes to respond to environmental conditions
Signal transduction begins when a dimeric integral membrane protein senses an external signal, and one of the monomers phosphorylates the other on a histidine residue. This phosphoryl group is transferred to the cytosolic response regulator protein, which initiates changes in gene expression
Salmonella typhimurium PhoPQ promotes virulence of this pathogen in humans, and the activity of the sensor component PhoQ can be repressed by divalent cations, such as calcium and magnesium
Summary
Two-component systems enable microbes to respond to environmental conditions. Signal transduction begins when a dimeric integral membrane protein senses an external signal, and one of the monomers phosphorylates the other on a histidine residue. Cho et al describe the structure of the external domain of PhoQ, with four monomers in the asymmetric unit of the crystal. Chemical and structural experiments pinpoint at least three calcium-binding sites per monomer on this surface, which would allow the dimer to lie flat on top of the negatively charged head groups of the lipid bilayer. This positioning leads the authors to propose that displacement of the divalent ions (for instance, by cationic antibiotic peptides) would trigger a lever-like movement of the sensor domain up off of the membrane surface, thereby turning on the kinase.
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