February 24, 2005
La Jolla, CA – The elusive world of membrane proteins – the crucial gatekeepers of a body cell’s outer wall that are popular targets for scientists trying to understand the molecular origins of health and disease – has been made more accessible through a discovery published in the February 25 issue of Science.
Researchers from the Salk Institute for Biological Studies have discovered a ‘partner’ molecule called Mistic that at last makes possible widespread production of membrane proteins so that they can be studied, allowing scientists to determine their atomic structure and design drugs that interfere with disease processes involving membrane proteins.
“The human genome encodes approximately 30,000 genes, one-third of which are devoted to coding integral membrane proteins,” said Senyon Choe, team leader of the study. “Out of approximately 10,000 human genes dedicated to integral membrane proteins, only a small handful of proteins have been made available from natural sources in the quality and quantity needed to study them successfully in isolation prior to this.”
Membrane proteins, such as receptors and ion channels, are tiny molecular ‘machines’ that are embedded in the walls of cells, which act as gatekeepers, filtering and passing messages between the inside of the cell and its outside, i.e., the rest of the body. Not surprisingly, they are crucially important in medical research and drug discovery, but until now human membrane proteins have been virtually impossible to produce in large enough amounts to study.
Proteins that exist freely inside the cell can be studied relatively easily once they are cloned by inserting genes into bacteria such as E. coli. By contrast, since membrane proteins are designed to be embedded in the cell membrane, they need an extra step to function properly – they need to be inserted into a cell membrane when they are created. There was no way to do this easily, until the Salk team discovered Mistic. Like a host drawing a reluctant guest into a cocktail party, Mistic appears to facilitate the integration of cloned membrane proteins into the E. coli cell membrane.
Using NMR spectroscopy Salk scientist Roland Riek led the efforts to determine Mistic’s unusual structure, which consists of a bundle of four helices, like two pairs of corkscrews, that appear to automatically fold into place within the cell membrane.
“We think that these four helices give Mistic a self-integrating capability,” said Riek. The Salk team speculates that this process draws their ‘guest’ protein into the E. coli membrane with them.
“It seems that Mistic inserts autonomously into the membrane and that this facilitates the rest of the molecule, the ‘cargo’ protein, to undergo the folding and integration process,” said Tarmo Roosild, lead author of the study.
The auto-inserting ability of Mistic gives medical researchers a new tool that could revolutionize membrane biology. It allows them for the first time to produce large quantities of crucial membrane proteins for structural study or therapeutic research, such as ion channels and the vast family of receptors called G-protein coupled receptors (GPCRs). Approximately 1,000 GPCRs exist, involved in a wide variety of body processes from vision to sexual development as well as many endocrinological and autoimmune disorders. Choe’s team has now successfully created dozens of such important human membrane proteins in the cell membrane of E. coli using Mistic.
More than half the blockbuster drugs in the pharmaceutical industry are targeting just two classes of membrane proteins: ion channels and GPCRs. “Figuring out how to produce them abundantly using Mistic is probably the most fascinating, pure breakthrough discovery we have ever made in my 12-year old laboratory at the Salk,” said Choe. “Mistic offers an unprecedented opportunity to answer the problems we have long been eager to tackle.”
The Salk Institute for Biological Studies in La Jolla, California, is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health and the training of future generations of researchers. Jonas Salk, M.D., whose polio vaccine, which was proven safe and effective in 1955, has eraadicated almost all cases of the crippling disease poliomyelitis, opened the Institute in 1965 with a gift of land from the City of San Diego and the financial support of the March of Dimes.