Proteins gone wrong cause most human diseases. Find these mutated proteins, scientists reason, and they are on the way to predicting who will get what disease. They would also learn scads of new biology to help doctors decide when to start available treatments, and to help in the search for additional treatments.
Of course, there’s a big problem to address first, or someone would have done this already. Experts estimate that humans have about 100,000 proteins in each of their cells. In the daily course of living, many of those proteins interact with each other. So researchers faced the gargantuan task of cataloging an incredible number of interactions – millions, if not billions.

“It’s a terrific challenge, but also a terrific opportunity,” says Marc Vidal, who heads a team that has begun to put together a Who’s Who in human proteins at the Harvard Medical School and the Center for Cancer Systems Biology at Dana-Farber Cancer Institute in Boston. The team has looked at 66 million combinations between 8,100 proteins and identified 2,800 interactions among them. The search revealed more than 300 new connections to more than 100 proteins associated with human disease.

“It’s a start,” says Vidal, an associate professor of genetics at Harvard Medical School. “We could not have done this even five years ago. Now, because of the Human Genome Project, we have a first-draft list of our genes, which carry the blueprints for all proteins.” Protein numbers, about 100,000, exceed those of genes, about 25,000, because some genes make more than one protein.

“Protein interactions are cumbersome to detect directly in a cell, so we work through their genes,” Vidal explains. “That way, we produced 8,100 proteins whose interactions we tested. These represent only a few percent of all the interactions that occur in human cells. But we have built a scaffold to which other interactions can now be attached.”