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But Groves thinks those results suggest something else: the structure and composition of the membrane are important for all processes. “I think it's really a mistake to think there are only two types of things in the membrane: rafts and everything else,” he says. “I think every process assembles its own object. Cholesterol is important, lipids are important. And a truly useful description of membrane organization needs to capture this diversity.”
To probe how the organization of groups of membrane proteins affects their function, Groves has developed a technique called spatial mutations. He creates artificial bilayers that impose organization on a cell, using miniscule tools designed for nanoengineers who build things even smaller than cells. His lab débuted the strategy in a paper on T cell-receptor signaling in Science in 2005. T cells are immune cells responsible for recognizing antigens in the body and eliciting a specific response. This process relies on a number of receptors clustered on a cell membrane that must work closely together.
When Groves's team used its artificial membranes to move the components of the T cell receptors around, the receptors stopped working normally. “There were no genetic mutations, and no drugs,” says Groves. “We just physically reorganized the membrane and could track differences in signaling.”
Membranes, says Groves, provide places where molecules can be organized in nonrandom ways, clustered where they're needed. His research continues to probe the membrane as a complete system, rather than as isolated proteins. “The single molecule approach will always be there, that's essential,” he says, “but we're starting to have tools to go after the collective as well. We want to see the forest and the trees both.”
Harvard's Walz shares this view. He has moved from studying the structure of aquaporins to looking at the broader view—probing how the channels interact with the membrane around them.
His lab has been testing the theory that helix-shaped parts that exist in many membrane proteins, including aquaporins, can adjust to the bilayer surrounding them by expanding and contracting like a spring. If they're in a thick lipid membrane, the helices can stretch a little bit, and if they're in a thin spot, the helices will condense.
“That is just a theory, and using aquaporins we can actually measure whether this is happening or not,” says Walz. By doing crystallography of aquaporins while they're embedded in the membrane, as he and Gonen did for the AQP-0 structure, Walz can see how the structure changes in membranes of varying widths.
Innovative experiments like this, membrane scientists agree, will answer the questions raised by a decade of structural work.
“It's a little like the stage of biology back when collectors were going out and collecting things and sending them back to museums,” says Rod MacKinnon. “We have a large collection now. Which means we have to start figuring out the logic to how this all fits together.”