Sleuths solve one puzzle in the mystery of bacterial persistence

Photo by Cal­i­fornia Cthulhu (Will Hart) via Flickr

This after­noon, reading through Pro­fessor Kim Lewis’ soon to be pub­lished article in Cell Press (avail­able ahead of print here), I may have fan­cied myself some­thing of a pri­vate inves­ti­gator with the high stakes job of pro­viding a com­pre­hen­sive pic­ture of his new find­ings for you, my ded­i­cated reader. It was a pretty action-​​heavy couple of hours, despite the apparent silence in my lonely office. There were high-​​speed chases, but no cars were involved. Only pro­tein kinases and antibi­otic resistance.

Now that it’s all over, I think I have a good package to present you with. While I don’t have a manila enve­lope full of sleuthed photos taken with a wide angle lens, I hope you’ll still be convinced.

So, here’s what I found out:

Bac­teria, as you may know, have a ten­dency to develop antibi­otic resis­tance. How they do it is a mys­tery that has com­pelled actual sci­en­tific inves­ti­ga­tors for decades. In 1983, one team of said inves­ti­ga­tors iden­ti­fied a pro­tein crit­ical to what Lewis calls “per­sis­tence” — the ability of a bac­te­rial cell to “sleep” through an antibi­otic attack and “wake up” unharmed, ready to infect the host anew.

Under normal growth con­di­tions, HipA, as this pro­tein is called, is turned off by virtue of being bound to another pro­tein called HipB (obvi­ously). When HipA gets turned on, it starts run­ning around the cell snatching up pro­teins impor­tant to cell growth and turning them off, which puts the cell into a kind of hibernation.

Pre­vious work had sug­gested that HipA gets turned on when a cer­tain “sweet spot” is tagged with a chem­ical group called a “phos­phate.” But this didn’t make much sense because get­ting a phos­phate to that sweet spot was like trying to mix oil and water.…literally: the spot is buried in a hydrophobic core while the phos­phate is hydrophillic.

Lewis’ group decided to take a closer look at pHipA (HipA tagged with a phos­phate group). Just like zooming in to find Jes­sica Rabbit playing patty-​​cake with Marv Acme, this revealed more than they were expecting.

In this bound form, that sweet spot is actu­ally pulled out of the hydrophobic core, exposing it to an area much friend­lier to water-​​loving stuff like phos­phates. That explained how HipA got phos­pho­ry­lated in the first place, but it cre­ated a whole new mys­tery: now, instead of being buried in a vat of oil, so to speak, the sweet spot com­pletely blocks the active site making it impos­sible for HipA to do it’s job. Instead of being the “on” ver­sion of the pro­tein, pHipA is actu­ally another kind of “off” ver­sion, con­trary to what researchers had been thinking.

This new under­standing of the protein’s struc­ture allowed Lewis’ team to come up with a scheme for how HipA reg­u­lates persistence:

First, HipA is turned off because it’s bound to HipB. The bac­te­rial cell grows and repro­duces nor­mally. Then, for one reason or another, HipA breaks away from HipB and becomes active, sending the cell into hiber­na­tion. The sweet spot swings in and out of the hydrophobic core and if it hap­pens to get tagged with a phos­phate group, that shuts down HipA’s activity and the cell is able to wake up again.

It’s still unclear exactly what hap­pens after HipA gets phos­pho­ry­lated. Does it break down com­pletely or can it grad­u­ally get de-​​phosphorylated? This is where more work will need to be done.

Which is good, because I think I’ve filled you with enough evi­dence for one post. Stay tuned for more adven­tures in the life of this inter­na­tional woman of mystery…