Bacteria have RNAs that sense fluoride, and channels that tranport it

Fluoride: unless you’re a synthetic chemist or a dentist, you probably don’t worry about this ion very often.  But, according to a new paper published in Science, bacteria do, and have done for a very long time.

The work, spearheaded by Ron Breaker’s group at Yale University, identified a novel RNA motif that selectively binds fluoride ion.  In response to Fbinding, this motif, called a riboswitch, undergoes a structural change that leads to increased transcription of downstream genes.  These genes encode crucial metabolic enzymes that are strongly inhibited by fluoride ion, like enolase and pyrophosphatase, as well as members of a family of chloride transport proteins, the CLC’s.  The CLC’s that are associated with F riboswitches are clustered together in a phylogenetic clade distant from well-characterized CLC’s.  Could these “chloride” channel proteins actually assist with fluoride export?  Randy Stockbridge, a Brandeis postdoc working in Chris Miller’s lab, contributed to the findings by showing that this subset of riboswitch-associated CLC’s do, in fact, transport F, whereas “conventional” CLC’s strictly exclude F.   The F riboswitches, and the F CLC’s, are found among a huge variety of bacteria and archaea, from plant and human pathogens to benign soil and seawater-dwelling bugs, leading to the inference that F toxicity has been a consistent evolutionary pressure.

You’re probably wondering just how much fluoride there is in the environment.  Fluoridated municipal drinking water contains about 80 micromolar F, and natural F- concentrations in the environment can be  higher and lower than that number.   In acidic environments especially, F might accumulate to much higher levels in bacteria.  With a pKa of 3.4, a small amount of F is present as HF at low pH, and the uncharged HF can diffuse cross the cell membrane into the cell.  Once in the cytoplasm, where the pH is around 7, HF dissociates, and F can’t diffuse across the membrane back into the environment.  Unless, of course, evolution has provided that bacterium a system to transport F out of the cell…

see also

For ClC transporters, breaking up is hard to do

Many ion channels and transporters exist as oligomers with each subunit containing a distinct transport pathway.  A classic example is the ClC family of chloride channels and transporters that are homodimeric with a pathway for chloride permeation or chloride/proton anti-port through each subunit.  Because of their dimer structure, they have come to be known as “double-barreled shotguns” for chloride movement across the membrane.

Since each subunit appears to possess the complete machinery required for transport, it is  often wondered whether ClCs need to be dimeric in order to carry out function.  In a study published last week in Nature, Brandeis researchers Janice Robertson, Ludmila Kolmakova-Partensky and Professor Christopher Miller answer this question.  By introducing two tryptophan mutations at the dimer interface, they designed a variant of a ClC transporter that could be purified and crystallized as an isolated monomer.  With this, they were able to determine that the monomer alone was fully capable of carrying out chloride and proton transport function.  These results show that the dimer is not required and that the monomer is the fundamental unit of transport in ClCs.  The question of why ClCs evolved as dimers remains a key question for understanding membrane protein structure.

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