The physics of chaperones

A simple physical property based on entropy gives Hsp70 a pulling force, say Paolo De Los Rios (EPFL, Lausanne, Switzerland), Pierre Goloubinoff (University of Lausanne, Switzerland), and colleagues. The new theory resolves two deadlocked models. 
 
 
 
Figure 1 
 
Adding Hsp70 reduces mobility and drives motion away from the membrane. 
 
 
 
Hsp70s have two major activities: they help unfold stable protein aggregates, and they pull proteins through membrane channels. In the Brownian ratchet model for protein import, Hsp70 grabs part of a protein once it is spontaneously unfolded by random thermal fluctuations, and passively prevents it from sliding backward. By contrast, the power stroke model has the Hsp70 using a channel protein as the fulcrum for a lever arm movement that yanks the protein inwards. 
 
The Lausanne group suggested that an emerging protein with a bulky Hsp70 attached would keep bumping against the nearby membrane. But if the whole complex moved further away from the membrane, this bumping no longer happened. The resulting increase in mobility and thus entropy meant a favorable change in free energy—a change that powers Hsp70's pulling force away from the membrane. Similar physics would draw Hsp70-bound protein segments away from the bulk of an aggregate and thus tease apart the tangles. 
 
“We made a chimeric theory,” says Goloubinoff. “Yes, there is active pulling, but there is no lever.” The Brownian ratchet remains partly intact also, as the unfavorable change of free energy forbids Hsp70 from being pushed back toward the membrane. 
 
“The physics behind it is very simple,” says statistical physicist De Los Rios. “It's been known that you can generate forces on polymers when you manipulate the number of conformations that the polymer can exist in.” With the Hsp70 case, he worked out how much work this change could do and found it was compatible with the forces required for protein unfolding and accelerated import. 
 
In theory, this leap in thinking could have been made at any time in the last 20 years. In fact, says De Los Rios, “the first thing I said is, this is a nice application, but I asked who must I cite. But I couldn't find anything.” Goloubinoff suggests a precedent. “When I was doing my Ph.D. we had all the tools of molecular biology,” he says. “But there was only one person who thought of PCR.” 
 
Reference: 
 
De Los Rios, P., et al. 2006. Proc. Natl. Acad. Sci. USA. doi:.10.1073/pnas.0510496103 [PubMed] [Cross Ref]

A simple physical property based on entropy gives Hsp70 a pulling force, say Paolo De Los Rios (EPFL, Lausanne, Switzerland), Pierre Goloubinoff (University of Lausanne, Switzerland), and colleagues. The new theory resolves two deadlocked models.
Hsp70s have two major activities: they help unfold stable protein aggregates, and they pull proteins through membrane channels. In the Brownian ratchet model for protein import, Hsp70 grabs part of a protein once it is spontaneously unfolded by random thermal fl uctuations, and passively prevents it from sliding backward. By contrast, the power stroke model has the Hsp70 using a channel protein as the fulcrum for a lever arm movement that yanks the protein inwards.
The Lausanne group suggested that an emerging protein with a bulky Hsp70 attached would keep bumping against the nearby membrane. But if the whole complex moved further away from the membrane, this bumping no longer happened. The resulting increase in mobility and thus entropy meant a favorable change in free energy-a change that powers Hsp70's pulling force away from the membrane. Similar physics would draw Hsp70-bound protein segments away from the bulk of an aggregate and thus tease apart the tangles.
"We made a chimeric theory," says Goloubinoff. "Yes, there is active pulling, but there is no lever." The Brownian ratchet remains partly intact also, as the unfavorable change of free energy forbids Hsp70 from being pushed back toward the membrane.
"The physics behind it is very simple," says statistical physicist De Los Rios. "It's been known that you can generate forces on polymers when you manipulate the number of conformations that the polymer can exist in." With the Hsp70 case, he worked out how much work this change could do and found it was compatible with the forces required for protein unfolding and accelerated import.
In theory, this leap in thinking could have been made at any time in the last 20 years. In fact, says De Los Rios, "the fi rst thing I said is, this is a nice application, but I asked who must I cite. But I couldn't fi nd anything." Goloubinoff suggests a precedent. "When I was doing my Ph.D. we had all the tools of molecular biology," he says. "But there was only one person who thought of PCR."

SORGER/ELSEVIER
Adding Hsp70 reduces mobility and drives motion away from the membrane. Territories were so named based on microscopists' early observations: that individual chromosomes did not spread out over an entire nucleus. DNA labeling also indicated that nuclei had gaps between DNA, although it was not clear if these were between DNA of the same or different chromosomes. Still, this was enough to spark talk of an interchromatin domain separating territories. "In this model, it will always be right because two pieces of chromosome will never be in exactly the same place," says Pombo. "The fi eld couldn't move forward." She and Branco responded by labeling two different chromosomes in ultrathin slices and following up with EM for even higher resolution. Based on extrapolation, an average of 46% of each chromosome territory intermingles with other chromosomes. Furthermore, extents of intermingling for different chromosomes correlated with translocation frequencies.

The physics of chaperones
The nucleus is not a complete tangled mess, however. "Chromosomes as polymers will tend to expand," says Pombo. "But what stops them is interaction with each other and with the nuclear membrane." She is now seeing if transcription factories help keep particular chromosomes fi xed to themselves or each other. Immune to weight loss L eptin always seemed a bit too good to be true. Why would our chronically undernourished ancestors have evolved a hormone for weight loss? Now, Ke Chen, Allan Zhao (University of Pittsburgh, PA), and colleagues explain why leptin doesn't work as the weight loss wonder drug that initially seemed a possibility.
Leptin was discovered as a hormone produced by fat cells. When reinstated in leptin-defi cient mice and humans it drastically reduced their obesity. But individuals who were obese for other reasons had paradoxically high leptin levels, and adding more leptin did not induce signifi cant weight loss.
The Pittsburgh group looked to leptin-binding proteins for an answer. The fi rst that they found was C-reactive protein (CRP), which is famous as a marker for immune reactions but whose exact function remains mysterious. CRP bound to leptin, and inhibited leptin signaling in tissue culture. In mice lacking their own leptin, CRP reduced or eliminated the effects of added leptin on food intake, body weight, blood glucose, and lipid metabolism.
The CRP connection is not completely without precedent, as others have noted infl ammation (and elevated CRP) as a complication of obesity. The adipogenic and infl ammatory systems also share many of the same mediators, so there is plenty of potential for crosstalk. It is not clear whether this is an accident of evolution or has a particular function.
Zhao's one clue is that low levels of leptin turn on CRP expression. He suspects CRP helped our ancestors to accumulate some fat in the few periods of plenty interspersed amongst the more common lean times. "If leptin worked unimpeded you would have a very diffi cult time accumulating fat," he says. Now that energy rich food is always available, however, the system has become pathological. Zhao's next step is to fi nd out whether those who are morbidly obese have a different set point for CRP expression or accumulation. D uring heat shock recovery, a helper becomes a destroyer, say Shu-Bing Qian, Cam Patterson, and colleagues (University of North Carolina, Chapel Hill, NC). The traitor protein is called CHIP. After the group discovered CHIP, their interest was piqued when they realized it was both an Hsp70-binding protein and a ubiquitin ligase. The fi nding suggested that "folding versus degradation is a regulated decision, not a stochastic decision, and CHIP ties together these two processes," says Patterson.
They now fi nd that CHIP helps to mark unfolded proteins that are bound to Hsp70 by adding the ubiquitin destruction signal. But once those marked proteins are destroyed, the Hsp70 itself becomes CHIP's next target. The effect could be reconstituted in vitro with only unfolded protein, Hsp70, and CHIP.
The resultant return to baseline Hsp70 levels is important to restore cellular normality. Hsp70 helps halt the cell cycle and alter signaling pathways so that cells can recover from stress. Once unfolded proteins are cleared, says Patterson, "the cell has to get out of this suspended animation phase."