Researchers have determined the structure of part of the tiny passageways that allow calcium ions to enter mitochondria and kick off cellular energy production.

The findings, reported May 2 in Nature, promise to help researchers better understand how the channel, known as the mitochondrial calcium uniporter, works so speedily and precisely and what happens when it breaks—a question of growing interest since mutations in the uniporter have recently been linked to intermittent fatigue and lethargy.

“The uniporter has been a puzzle since its discovery more than half a century ago,” said the study’s senior author, James Chou, professor of biological chemistry and molecular pharmacology at Harvard Medical School. “It’s ancient, but it’s complex. We wanted to know what it looked like because that could help us understand how it works and how others might want to target it with drugs.”

Chou, who specializes in deciphering protein structures, partnered with Vamsi Mootha, professor of systems biology at HMS and professor of medicine at Massachusetts General Hospital, who focuses on mitochondrial biology and disease.

Mootha’s lab had identified all the molecular components of the uniporter in a series of studies between 2010 and 2013, including the pore subunit where calcium gets drawn in, called MCU. He was looking for someone to illuminate the channel’s architecture.

“James’ achievement represents a major advance for the field,” said Mootha, who is a coauthor on the paper. “It offers the first structural blueprint with which to understand the pore.”

Too big to fail

To sketch that blueprint, Chou’s team first had to do some tricky biochemistry to generate the amount of functional MCU needed for a structural study. Ultimately, they coaxed bacteria to produce unfolded MCU molecules. They then harvested the molecules and refolded them.

“We didn’t think we could refold MCU, but it worked,” Chou said.

Another problem loomed: MCU seemed to resist crystallization and was too large for nuclear magnetic resonance.

The team tackled the challenge by chopping off a part of MCU that Mootha’s experiments indicated was dispensable for studying pore formation. The truncated version of MCU still pushed the limits of what NMR could handle, “but it was within our reach,” said Chou. “We just went for it.”

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