New Metabolic “Switches” Discovered

Researchers from the Howard Hughes Medical Institute and Duke University Medical Center have identified a new class of metabolic switches, called G proteins, in yeast, which if found to be conserved in humans, could lead to the development of new drugs for treating diseases including diabetes, alcoholism, and heart disease.

The study, which was funded by the National Institutes of Health and the Burroughs Wellcome Fund, appears in the July 19, 2002, Molecular Cell. It is co-authored by HHMI investigator Dr. Joseph Heitman, M.D., Ph.D. and Toshiaki Harashima, Ph.D., with the Duke Center of Microbial Pathogenesis and HHMI at Duke.

G proteins are key controllers of the body’s internal “switchboard” of metabolic pathways. They typically lie just inside the cell membrane, attached to “G protein-coupled receptors” (GPCRs) on the cell surface, which respond to external chemical signals such as hormones. Once such an external signal activates a receptor, it switches on its coupled G protein, which in turn triggers a cell response. G proteins control such cellular responses in tissues throughout the body, including the heart, lungs, adrenal glands, liver, brain and other organs. G proteins malfunction in humans can lead to symptoms associated with diabetes, alcoholism, cholera and whooping cough.

To date, scientists have reported approximately 450 genes for G proteins. Metabolic pathways involving the receptors for such G proteins are the targets of hundreds of drugs, including antihistamines, neuroleptics, antidepressants and antihypertensives. At least 50 percent of all drugs sold today target G protein-coupled receptors that couple to G proteins. However, the functions of many of these proteins are unknown. In humans, there are more than 1,000 types of G protein-coupled receptors in the brain, which indicates the great potential for drug discovery by studying GPCRs and their associated G proteins, said Heitman.

“This novel class of G proteins, which if proven to be conserved in humans, could play a role in allowing our cells and bodies to sense unique signals important in both health and disease,” Heitman said.

The researchers’ study focused on a yeast G protein-coupled receptor called Gpr1 that is coupled with a G protein called Gpa2. Functionally, the Gpr1 receptor senses glucose in the yeast cells’ environment and activates the coupled Gpa2 to launch a growth process in which the yeast cells elongate and produce filaments that extend away from the colony and into the growth medium to forage for nutrients.

G proteins are “heterotrimeric” complexes, meaning that they are composed of three different subunit proteins called alpha, beta, and gamma — each of which plays a role in the transmission of the metabolic signal to the cell’s machinery. Since Gpa2 is highly related to the alpha subunit, the researchers expected to find associated beta and gamma subunits — but they were not present. According to Heitman, absence of these two subunits raised questions of whether the GPa2 functioned alone, or whether there existed as-yet-undiscovered classes of G protein subunits.

“Consider the analogy of a relay race,” Heitman said. “To run a relay you typically need four runners or swimmers. If any one is missing, the baton cannot be passed. So in this signaling pathway, who was passing the baton from the first runners to the last runners? Were they skipping a runner, or was there a novel runner that we didn’t know about?”

Those questions led Heitman and his colleagues to identify three novel G protein subunits — two closely related subunits called Gpb1 and Gpb2, and a third called Gpg1.

The newly discovered subunits offer an example of completely unrelated proteins with similar biological functions, Heitman said. Proteins are made of strings of amino acids that once synthesized, fold into the complex globular shapes that make them into functioning enzymes. Proteins that perform similar functions typically share the same amino acid sequences. In this case, even though the Gpb1 and Gpb2 proteins function like G protein beta subunits, they still lack any known amino acid sequence similarity to beta subunits.

Using biochemical and genetic analyses, the researchers found that Gpa2 plays an activational signaling role, which means Gpa2 functions as a molecular gas pedal to turn the pathway on. On the other hand, Gpb1 and Gpb2 subunits play inhibitory signaling roles, which means these proteins function as molecular brakes to constrain signaling between the Gpa2 protein and another unknown target in the metabolic pathway. Also, the researchers found that the Gpg1 subunit appears to interact indirectly with Gpa2 subunit, through the Gpb1 or Gpb2 subunits.

Intriguingly, the researchers found that the Gpb1 and Gpb2 subunits contain very divergent repetitive sequences of amino acids in key sections of their structures, compared to G protein beta subunits; however once they fold into their working shape, they still function similarly. Such evolution of different molecules or structures to have similar function is known as “convergent evolution.”

“Because these two completely different kind of repeat protein families fold into a similar structure, this suggests a striking example of convergent evolution with related structures. Which further suggests there might be two structurally related, but sequence divergent families of heterotrimeric G proteins,” Heitman said.

Heitman and colleagues will continue to study the structures of the newly discovered G proteins. They will also attempt to identify the molecular targets of the G proteins and study whether the proteins are conserved in multicellular organisms, such as insects, plants, and animals.