Layout, Workings Of Drug Target Found

The enzyme mTOR is the target of an approved immunity-suppressing drug as well as drug candidates for cancer and other conditions. Some scientists have longed to know its structure, how it interacts with substrates, and how it catalyzes reactions so they could use the information to develop safer and more powerful mTOR-targeted medications. But it’s been difficult to obtain sufficient material suitable for structural analysis.

Nikola P. Pavletich and coworkers at Memorial Sloan-Kettering Cancer Center have now succeeded in obtaining crystals and have analyzed mTOR structurally and mechanistically (Nature 2013, DOI: 10.1038/nature12122). The work suggests that one of mTOR’s protein domains is a gatekeeper that controls substrates’ access to the enzyme’s active site.

mTOR plays a critical role in both cancer and immunity by stimulating production of components needed for protein synthesis and the growth and division of cells, including cancer and immune-system cells. The approved drug that interacts with mTOR is the immunosuppressant rapamycin (sirolimus), a microbial natural product that prevents people’s immune systems from rejecting transplanted kidneys, livers, hearts, and other organs.

In 1994, two groups, led by Harvard University chemistry professor Stuart L. Schreiber and Johns Hopkins University neuroscientist Solomon H. Snyder, discovered that rapamycin and a protein called FKBP12 collaborate to bind mTOR and inhibit its activity. The result of those studies was the discovery and naming of mTOR (mammalian target of rapamycin).

“My first impression is ‘Wow!’ ” says Schrei­ber of the new study. “I’ve been waiting for years to see these insights.” The work by Pavletich and coworkers “fills in critical gaps in our understanding of a remarkable feat of natural selection”—the engagement of a microbial product and a small protein to form a composite surface that targets the huge mTOR and shuts down its operation, he says.

In a Nature commentary, Dario R. Alessi and Yogesh Kulathu of the MRC Protein Phosphorylation & Ubiquitylation Unit at the University of Dundee, in Scotland, point out that the function of mTOR in cells is to catalyze the phosphorylation of signaling molecules, which sends signals that tell cells to grow and multiply. “mTOR integrates and interprets all sorts of factors that influence cell growth—including nutrients, stressors, and the outputs of signal-transduction networks—by targeting a multitude of substrates that drive processes such as protein translation, metabolism, and cell division,” they explain.

“Research into mTOR-mediated signaling has taken on added urgency since it was discovered that most cancers contain mutations that inappropriately activate this protein,” they add. That’s why mTOR is a cancer target.

Pavletich and coworkers coaxed human cells to produce mTOR, assembled it with a stabilizing subunit, and identified a truncated version they could crystalize and analyze. The structure enabled them to propose how mTOR works and how its inhibitors achieve their potency and selectivity.

“The solution to the problem is beautiful and elegant,” Schreiber says. Normally, mTOR’s FRB (FKBP12-rapamycin binding) domain “dangles in front of the kinase domain, luring into the active site certain mTOR substrates.” When FKBP12-rapamycin binds FRB, however, the domain “clamps down on the enzyme active site, blocking access to substrates and preventing FRB from playing its normal substrate-recruitment role,” Schreiber says. He adds that he would now like to explore new ways to achieve the exquisite target selectivity of small molecules like rapamycin “by exploring ideas that result directly from this important advance.”