Kawate lab unveils structure of major membrane protein

In a paper published on Feb 12 in the journal eLife, Dr. Toshimitsu Kawate, associate professor in the Department of Molecular Medicine and his team reveal the form of pannexin1, a cellular membrane protein present in all vertebrates. With the size, shape and formation of this protein revealed for the first time, scientists can get closer to fine tuning new therapies for a multitude of diseases, including chronic pain, infertility and cancer.

Pannexin 1 is a channel protein — a gatekeeper that sits in the cell membrane, selectively allowing molecules to pass in and out of the cell. Its primary cargo that it allows through is ATP.  While ATP is often thought of as the energy currency of the cell, it is a major messenger molecule throughout the body. Knowing how that ATP message is delivered is important — it is integral to several conditions that are cause by too much, or not enough ATP signaling. Thus, Kawate was keen to learn the shape of Pannexin 1.

His lab used cryogenic electron microscopy (cryo-EM) to capture the protein — a technique that uses electrons, rather than light, to capture an image of almost atomically-sized things. Up until now, cryo-EM wasn’t refined enough to image something as small as a channel protein. But, thanks to the rapid development of the technology in recent years, Kawate was able to finally get his snapshot.

The Kawate research group.

“This is the first time we’ve actually seen the structure,” says Kawate. “We thought its overall shape would be similar to other channel proteins, but it wasn’t.” Most channel proteins are shaped roughly like a cylinder made up of subunits. Other channel proteins have a nice even number of subunits — six or eight. Pannexin 1, however, was a black sheep. “This is the first heptamer — a seven-piece channel discovered in eukaryotes,” says Kawate. “That’s one of the biggest surprises we had.”

Kawate believes this odd structure may be due to a type of ‘Goldilocks’ effect.  Channels with six subunits are too small to let ATP through, whereas channels with eight subunits are too big to be selective for ATP. But seven may be just right.

The heptamer structure wasn’t the only surprise for Kawate. As he dug into the molecular building blocks of the channel, he found something odd at the narrowest point of its cylindrical structure. At this point — the bottleneck of the channel, he had expected to find molecules that attract and aid the passage of ATP, Pannexin 1’s main cargo.  Instead he found tryptophan—a large amino acid that, according to Kawate, does not help draw ATP through the bottleneck at all. In fact, tryptophan may actually hamper the passage of ATP.  “That was actually shocking,” he says.

As puzzling as it is, Kawate notes it’s only part of the picture. The cryo-EM image only captures part of the full channel protein. The remaining portion — which is flexible and hard to image, could answer some of the remaining questions around how Pannexin 1 manages to ferry ATP through its channel.

With this latest discovery only revealing more mysteries, Kawate isn’t waiting to continue his investigation of Pannexin 1, which involves capture a full, untruncated image of the channel. With a full understanding of this channel protein, new insights and possibly even therapeutics, may follow.

-By Lauren Cahoon Roberts