Table of Contents
Beyond Genes:
Lipid Helps Cell Wall Protein
Fold into Proper Shape
William Dowhan, Ph.D.
A protein that provides a vital passage through a bacterium’s outer cell wall will misfold and malfunction if that wall is built of the ‘wrong’ material, scientists at The University of Texas Medical School at Houston have reported in a finding that has long-term implications for understanding diseases caused by misfolded proteins, such as cystic fibrosis, Alzheimer’s disease and mad cow disease.
The paper in the July 15 issue of the Journal of Biological Chemistry by William Dowhan, Ph.D., holder of the John S. Dunn Sr. Chair in Biochemistry and Molecular Biology, and colleagues shows that phospholipids, which make up the permeable barrier of cell membranes, play a direct role in the folding of membrane proteins – proteins that penetrate the membrane or bind to either side of it.
“What we’ve demonstrated again is that it’s not just a membrane protein’s genetically determined sequence that dictates how it folds so that it can function properly. Its lipid environment also plays a role,” Dowhan said. “People used to assume that specific lipids made no difference.”
In the paper, Dowhan and colleagues looked at how a protein called GabP, which transports an amino acid across the membrane of the bacterium E. coli, is affected by the presence of a phospholipid named phosphatidylethanolamine, or PE for short.
Phospholipids, unlike their fatty acid and cholesterol cousins, include a phosphate group that spurs them to form a bilayer with water-friendly outer layers sandwiching a water-unfriendly inner layer that defines the outer surface of cells. Transport of nutrients and waste material across the cell membrane is then governed by the specific proteins associated with it.
In a strain of E. coli lacking PE, the GabP protein misfolded, with two areas of the protein inverting from their normal structure. The PE-lacking protein’s amino acid transfer rate plummeted to nearly zero, falling 99 percent compared to the transfer rate in unaltered E. coli with PE.
GabP is the third membrane protein that Dowhan and colleagues have shown to be affected by the presence of PE.
The team is using the E. coli model to discover how all proteins fold in the membrane, not just transport proteins, such as GabP, but also biosynthetic proteins that manufacture complex compounds, such as proteins and fats, out of simple compounds.
“The next goal, now that we’ve defined the phenomenon, is to get into the specifics, find the mechanisms by which these proteins fold. What part of the protein interacts with the lipid, and what part of the lipid with the protein?” said Dowhan, who is also on the UT Graduate School of Biomedical Sciences at Houston faculty.
Dowhan Research Recognized for MERIT |
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This summer, William Dowhan’s research earned an honor coveted among biomedical researchers everywhere. When his grant came up for renewal this past summer, the National Institute of General Medical Sciences (NIGMS) of the National Institutes of Health (NIH) awarded it MERIT status, essentially renewing it to 2015 instead of the typical five-year renewal. MERIT (Method to Extend Research in Time) awards cannot be applied for by researchers. They are chosen by NIH institutes to support scientists whose research and productivity are distinctly superior and who are likely to continue to perform in an outstanding manner. In 1972 as a new faculty member of a brand new medical school, Dowhan applied to NIGMS to fund his research program – “Structure and Function of Membrane Proteins.” The grant has been renewed ever since, an exceedingly rare instance of longevity that makes it the university’s longest-running NIH grant. Dowhan, Ph.D., now is holder of the John S. Dunn Sr. Chair in Biochemistry and Molecular Biology at The University of Texas Medical School at Houston and a faculty member in the UT Graduate School of Biomedical Sciences at Houston. “Bill was way ahead of his time in recognizing that membrane proteins were a worthwhile challenge back when very few scientists were studying them,” said Rodney Kellems, Ph.D., chair of the Medical School Department of Biochemistry and Molecular Biology. “He committed to the field, and now there is much broader recognition that membrane proteins need attention. Bill has the experience, the tools, the know-how and insight to continue to advance the field. His MERIT award is recognition of that.” Dowhan, Kellems noted, has played an important role in the school’s Center for Membrane Biology, a multidisciplinary center based in the department of biochemistry that also taps the talents of faculty in Pathology and Laboratory Medicine and in Microbiology and Molecular Genetics. The leading-edge center is headed by John Spudich, Ph.D., professor and holder of the Robert A.Welch Distinguished Chair in Chemistry. In April, Dowhan received the prestigious American Society for Biochemistry and Molecular Biology Avanti Award in Lipids. |
Understanding the molecular basis for membrane protein folding will help researchers address serious diseases caused by misfolded proteins. “In cystic fibrosis, Alzheimer’s disease and mad cow disease, the dysfunctional proteins are associated with membranes,” Dowhan said.
Membrane proteins make up 30 percent of known proteins. Dowhan estimates another 40 percent are loosely tied to membranes. “So you are looking at possibly 70 percent of biology occurring at or in a lipid membrane surface,” Dowhan said.
Membranes and their surface proteins are accessible targets for pharmaceuticals, and most drugs target either membrane proteins on human cells or the membranes of pathogens.
Co-authors of the paper with senior author Dowhan are first author Wei Zhang, Ph.D., a former graduate student who is now a postdoctoral fellow at Stanford University; postdoctoral fellow Heidi Campbell, Ph.D., of the UT Medical School Department of Biochemistry and Molecular Biology; and Steven King, Ph.D, associate professor, Department of Integrative Biosciences at Oregon Health and Science University.
A second major research area for Dowhan explores the role of a membrane protein called cardiolipin in the mitochondria – the energy powerhouse of cells in more complex life forms. Decreased levels of cardiolipin are implicated in heart muscle cell death, called apoptosis.
Dowhan uses a yeast model, because knocking down cardiolipin in animals kills them. Yeast, he says, “aren't happy about it” but survive for experimentation.
Yeast cells, like human cells, have a distinct, membrane- bound nucleus that harbors the chromosomes. Bacteria, such as E. coli, lack a nucleus.
By Scott Merville, Public Affairs