New crystallography facility should shed light on mysteries of cell membrane
April 5, 1996
By Lynn Yarris, LCYarris@LBL.gov
A cell's outer membrane is a thin wall of fatty molecules embedded with
thousands of assorted proteins. Some of these proteins control the flow of
materials in and out of the cell. Others function as receptors, receiving
chemical signals or messages from other cells. Despite their importance, the
workings of these proteins remain a mystery because of the lack of detailed
information on their structure.
The standard scientific technique for studying the structure of biological
molecules is x-ray crystallography, in which a beam of x-rays is sent through a
crystal of protein, forming a diffraction pattern when the beam's photons are
scattered by the crystal's atoms. This technique, however, has seen limited use
in membrane protein research in part because of the difficulty in growing
crystals which strongly diffract. Obtaining useful images therefore requires
either a lot of sample material or more photons than conventional x-ray sources
can deliver.
"The exceptionally high flux and brightness of the ALS' beams means we can get
high resolution images without the need for so much sample material," says
Thomas Earnest, a biophysicist in the Structural Biology Division who is
working with SBD Director Sung-Hou Kim and ALS scientists and engineers to
construct the MCF. "ALS beams also will allow us to collect data from crystals
which diffract weakly or have large unit cell dimensions, both of which are
typically true for membrane proteins."
This month the ALS will undergo a four-week shutdown for the installation of a
38-pole wiggler magnet that will be the heart of the crystallography facility.
The wiggler will provide highly collimated x-rays ranging in wavelengths from
0.9 to 4.0 angstroms. Precise tuning of these photons within this range will
make possible the use of the multiple-wavelength anomalous diffraction (MAD)
technique that is ideal for images of proteins and other biological
molecules.
"We will be able to offer our users faster, higher quality data over a wider
dynamic range," says Earnest. "We will also offer them a choice of
crystallographic techniques with ultrafast data collection."
The high flux of the ALS crystallography beamline will place a heavy demand for
high-speed detector readouts. Otherwise, valuable data could go unrecorded. The
immediate step to avoiding this potential problem will be the use of CCD
(charged-coupled device) cameras that have a readout time of just under two
seconds. CCD cameras are well-suited for the study of macromolecular crystals
in a steady state because they acquire diffraction pattern images on a
photographic plate over a given exposure time. For future studies of dynamic
processes, a new "pixel" detector is now being developed that will provide
continual data readouts.
"The pixel detector is based on technology developed for the Superconducting
Super Collider," Earnest says. "Its frameless, event-driven readout system will
permit the collection of higher-quality data, faster than present detector
systems, with the opportunity to continuously monitor time-dependent processes.
This will enable us to image the changes that a protein molecule undergoes
during a biochemical reaction."
With a data collection area that could ultimately be as large as one square
meter, Earnest says the pixel detector will be large enough to image viruses
and other biological structures in addition to proteins. This detector is being
designed in collaboration with Jacques Millaud of the Engineering Division,
Howard Padmore of the ALS, David Nygren of the Physics Division, and
researchers at UC San Diego. They expect a quarter-scale version to be ready
sometime next year.
With funding from the Department of Energy, the University of California, and
private industry, the MCF has been designated an official national user
facility. In keeping with this designation, Earnest and Kim have organized a
participating research team (PRT) which will include researchers from various
UC campuses as well as private industry.
To support the PRT effort, the ALS will also be opening (early this fall) the
Structural Biology Support Facilities. Located on the second floor of the Light
Source, above the crystallography beamline, and in Bldg. 80, the support
facilities will provide state-of-the-art biochemistry and spectroscopy
resources, as well high-performance computer capabilities.
"The use of modern molecular biology and biochemistry, combined with
state-of-the-art technology at the MCF, is necessary to attack the important
and difficult problems presented in structural molecular biology," Earnest
says.
One of the most critical components of a living cell is the outer membrane that separates the interior of the cell from the outside world. The study of cell membranes and other biological structures should get a big lift
this fall from the Advanced Light Source (ALS) with the commissioning of
beamline 5.0, the Macromolecular Crystallography Facility (MCF).