Beyond genes - Lipid helps cell wall protein fold into proper shape
17 Jul 2005

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 report 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 today's Journal of Biological Chemistry by Professor of
Biochemistry and Molecular Biology William Dowhan, Ph.D., 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 JBC 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 an impermeable 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 holds the John S. Dunn Sr. Chair in
Biochemistry and Molecular Biology and is on the Graduate School of
Biomedical Sciences faculty.

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 JBC paper with senior author Dowhan are first author
Wei Zhang, Ph.D., a former graduate student who is now a post-doctoral
fellow at Stanford University, and post-doctoral 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 & Science
University.

Dowhan recently was granted a MERIT award by the National Institute of
General Medical Sciences of the National Institutes of Health.

These rare awards provide long-term grant support for scientists whose
research competence and productivity are distinctly superior and who
are likely to continue to perform in an outstanding manner, the NIGMS
notes.

MERIT (Method to Extend Research in Time) status essentially gives
Dowhan a 10-year renewal to 2015 on his longstanding NIGMS grant
"Structure and Function of Membrane Proteins" by providing back-to-back
five-year grants of $2.4 million apiece.

In April, Dowhan received the prestigious American Society for
Biochemistry and Molecular Biology Avanti Award in Lipids.

Scott Merville
scott.merville@uth.tmc.edu
713-500-3042
University of Texas Health Science Center at Houston
http://www.uthouston.edu

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