New Details of Tuberculosis Protein-Cleaving Machinery Revealed

May 11, 2010
These images show crystal structures of a Mycobacterium tuberculosis proteasome with its gate closed (left panels) and opened (right). To be degraded, a protein substrate has to pass through the opened gate to reach the interior chamber where protein carving occurs.

( -- Scientists looking for new ways to fight tuberculosis (TB) have their sights set on a structure essential to the bacterium's survival. Disabling this structure could kill the microbes in the infected host and thwart TB infections.

In a study appearing online May 11, 2010, in EMBO J, the journal of the European Molecular Biology Organization, scientists from the U.S. Department of Energy's (DOE) Brookhaven National Laboratory, Stony Brook University (SBU), and Weill Cornell Medical College describe new features of how this structure, known as a proteasome, is put together and how it works. These details could assist researchers working to develop anti-TB drugs.

", the bacterium that causes TB, infects one person in three worldwide, so finding new ways to battle this pathogen is a major public health priority —particularly in developing nations where active TB infections are endemic," said study co-author Huilin Li, a Brookhaven biophysicist and associate professor at SBU.

Earlier studies by Li and his collaborators revealed important structural details of the Mycobacterium tuberculosis proteasome, a piece of that carves up unwanted or damaged proteins, allowing the bacterium to evade a key defense of the human immune system. The team has even identified small molecules that might be incorporated into drugs to inhibit the proteasome. "The primary aim of this new study was to look at how the proteasome, comprised of 28 proteins, is constructed," said Li.

The scientists used Brookhaven Lab's (NSLS) — a source of intense x-ray, ultraviolet, and infrared light — and a cryo-electron microscope to take molecular-level snapshots of the proteasome at various stages of assembly. The studies revealed important intermediate steps and changes in the shapes of the components making up the completed structure.

The snapshots also reveal how one component in particular can inhibit the assembly process.

"Such detailed understanding of the assembly process might suggest novel approaches for developing anti-TB drugs by preventing the maturation of the proteasome," said Li. "This would be an alternative to the traditional approach of inhibiting the activity of the mature proteasome."

The researchers were also curious to find out how the Mycobacterium tuberculosis proteasome keeps the entrance to the protein-cleaving chamber shut.

"The fully constructed proteasome is literally a death chamber for cellular proteins, so the passage to the chamber has to be safely closed, and open only when necessary," Li explained.

Higher-level organisms, such as humans or yeast, also have gate-closed proteasomes to degrade unwanted proteins. In these cases, the gate closure mechanism is known and straightforward: Each of the seven end proteins is different, and they can assume different conformations, or shapes, to open and close the gate.

But in bacterial proteasomes, the seven end proteins are identical. "The question has been how the same sequence takes on the necessarily different conformations in order to seal the central pore," Li said.

Images taken by the scientists using x-ray beams at the NSLS reveal an asymmetric and tightly closed gate structure at the seven-fold symmetrical entrance. The scientists also snapped additional images showing that the gate structure retains some flexibility.

"This flexibility may be key to opening the gate to allow entry to proteins that need to be degraded," Li said. "Figuring out how to reduce the flexibility, and thus keep the gate permanently shut, could be yet another strategy in developing proteasome-targeting anti-TB drugs."

These new approaches are particularly attractive because the differences in assembly and gating mechanisms between human and TB proteasomes are more significant than the differences in the enzyme active sites that have been primary targets for drug development. As a result, drugs designed to inactivate these aspects of the TB proteasome would be less likely to also inhibit proteasomes in human cells.

TB Survival Mechanism

Most people infected with TB remain symptom-free because the bacterium is kept in check within immune system cells. These cells produce compounds such as nitric oxide, which scientists believe damage or destroy the bacteria’s proteins. If allowed to accumulate, the damaged proteins would kill the bacteria. But the TB proteasome, a protein-cleaving complex, carves up the damaged proteins, allowing Mycobacterium tuberculosis to survive, and possibly go on to cause active infections. Scientists looking for new anti-TB drugs are using the latest details about the structure and assembly of the TB to find ways to disable this action.

Related Stories

Recommended for you

Hibernating ground squirrels provide clues to new stroke treatments

November 17, 2017
In the fight against brain damage caused by stroke, researchers have turned to an unlikely source of inspiration: hibernating ground squirrels.

Age and gut bacteria contribute to multiple sclerosis disease progression

November 17, 2017
Researchers at Rutgers Robert Wood Johnson Medical School published a study suggesting that gut bacteria at young age can contribute to multiple sclerosis (MS) disease onset and progression.

Molecular guardian defends cells, organs against excess cholesterol

November 16, 2017
A team of researchers at the Harvard T. H. Chan School of Public Health has illuminated a critical player in cholesterol metabolism that acts as a molecular guardian in cells to help maintain cholesterol levels within a safe, ...

Prototype ear plug sensor could improve monitoring of vital signs

November 16, 2017
Scientists have developed a sensor that fits in the ear, with the aim of monitoring the heart, brain and lungs functions for health and fitness.

Ancient enzyme could boost power of liquid biopsies to detect and profile cancers

November 16, 2017
Scientists are developing a set of medical tests called liquid biopsies that can rapidly detect the presence of cancers, infectious diseases and other conditions from only a small blood sample. Researchers at The University ...

FDA to crack down on risky stem cell offerings

November 16, 2017
U.S. health authorities announced plans Thursday to crack down on doctors pushing stem cell procedures that pose the gravest risks to patients amid an effort to police a burgeoning medical field that previously has received ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.