1. CLATHRIN STRUCTURE REVEALS MOTIFS FOR SELF-ASSEMBLY
by Annette Greiner
(Contact: hwang@msg.ucsf.edu)

A team of researchers from the University of California, San Francisco, and MEMOREC Stoffel GmbH, working at the Macromolecular Crystallography Facility at the ALS, has determined a key structure of the protein clathrin. Clathrin plays an intimate part in the creation of vesicles in cells. In association with an adaptor molecule near a membrane receptor, it self-assembles into a polyhedral (soccer-ball-shaped) lattice of many clathrin molecules that coats a new vesicle as it forms. This self-assembly is fundamental to the functioning of a cell: both the intake of nutrients by endocytosis and the biogenesis of organelles rely on clathrin for vesicle formation and protein sorting. The team's results show the structure of the portion of the clathrin molecule that is responsible for assembly and suggest how similar coupling may work in other proteins.

Clathrin is a trimer that takes the shape of a triskelion--a figure with three similarly bent legs springing from a common center. (A highlight with images of clathrin can be seen on the Web at http://www-als.lbl.gov/als/science/sci_archive/clathrin.html.) When clathrin molecules assemble to form a polyhedral lattice, the legs of nearby triskelions hook around each other. The structure of a segment of the leg in the triskelion "hub" (the central half of the leg) was solved by multiple-wavelength anomalous diffraction (MAD) at Beamline 5.0.2. More traditional crystallography techniques could not be used because the structure of the clathrin molecule changes too easily when different heavy metals are added to the crystal lattice to determine phases. For MAD phasing, the researchers substituted only selenomethionine for all of the methionine in the protein and collected diffraction data at four different wavelengths near the selenium K edge. The high intensity of the ALS light gave excellent diffraction data, from which the scientists were able to produce a high-quality electron-density map of the protein.

Refinement and analysis of the electron-density map showed that the triskelion leg segment contains a series of alpha helices joined by hairpin turns and loops (helix-turn-helix-loop). This finding confirms circular dichroism spectra that indicated a structure composed of alpha helices. Contrary to earlier predictions, however, the helices are oriented perpendicular, not parallel, to the major axis of the molecule. Superposition of the sequence for the hub section of the triskelion leg onto sequences further from the center, matching amino acid side chains, showed that each leg contains seven ten-helix repeats, dubbed clathrin heavy-chain repeats (CHCRs). Along the length of a CHCR, the helices are alternately oriented to one side or the other of the molecule, creating two major faces of helices and an edge of hairpin turns between them. The researchers postulate that faces in the legs of neighboring triskelions associate with each other to form the polyhedral clathrin lattice. In addition, histidine residues in the hairpin turns could stabilize the association.

The clathrin trimer consists of three heavy-chain subunits (the legs), each bound to one light-chain subunit in the center. The researchers found structural evidence that the superhelix plays a role in trimerization as well as assembly, and they identified a potential site for binding to the light chain in the innermost of the seven CHCRs. In addition, their sequence analyses of homologous proteins showed that the 10-helix repeat appears in other proteins involved in maintenance of vacuoles and protein sorting, suggesting a common coupling mechanism.

Research conducted by J.A. Ybe, F.M. Brodsky, K. Lin, S.-H. Liu, L. Chen, R.J. Fletterick, and P.K. Hwang (University of California, San Francisco); K. Hofmann (MEMOREC Stoffel GmbH); and T.N. Earnest (Berkeley Lab), using Beamline 5.0.2. Funding: National Institutes of Health, U.S. Department of Energy.

Publication about this experiment: J.A. Ybe et al., Nature 399 (1999), 371-375.

2. USERS' MEETING ABSTRACT DEADLINE EXTENDED TO SEPTEMBER 10
(Contact: alsabstract@lbl.gov)

The deadline to submit abstracts for the 1999 ALS Users' Meeting has been extended to Friday, September 10, 1999. The abstracts, along with oral and poster presentations given at the meeting, will be used to select the recipients of two ALS prizes: the Student Poster Award, for the best presentation about work conducted in the course of postgraduate studies, and the Shirley Award, for outstanding scientific achievement at the ALS. The abstracts will also be used to select invitees to give oral presentations.

If you wish to submit a one-page abstract detailing research done at the ALS over the past year, now is the time to do so. At the bottom of the page, include the name, postal address, email address, phone number, and fax number of the primary author. Please write "student" if applicable and "poster only" if the submission is not to be considered for an oral presentation.

Send abstracts by September 10, 1999, to
Ruth Pepe
Advanced Light Source, MS 6-2100
Berkeley Lab
Berkeley, CA 94720

Phone: (510) 486-5268
Fax: (510) 486-4773
Email: alsabstract@lbl.gov

3. MACROMOLECULAR CRYSTALLOGRAPHY BEAMTIME PROPOSALS DUE SEPTEMBER 15

The User Services Office is accepting proposals from scientists who wish to conduct research as independent investigators at the Macromolecular Crystallography Facility (Beamline 5.0) between January and June 2000. The deadline for submissions is September 15, 1999. There will be no automatic rollover of proposals from the previous proposal cycle (July to December 1999). Scientists wishing to renew a previous proposal should notify the ALS User Services Administrator, Ruth Pepe (contact information below).

The proposal form for independent investigators is available in Portable Document Format (PDF) on the Web (http://www-als.lbl.gov/als/quickguide/independinvest.html). Information on the proposal process is available at the same location. A data sheet on the Macromolecular Crystallography Facility provides information that may be useful to prospective users. It is available on the Web as a PDF file at http://www-als.lbl.gov/als/als_users_bl/5.0-Datasheet.pdf. If you do not have Web access and would like to request a data sheet, send an email request to alsuser@lbl.gov. Beamline information is also available on the Web in HTML format at http://www-als.lbl.gov/als/als_users_bl/bl_table.html.

To request a proposal form by mail, contact
Ruth Pepe, ALS User Services Administrator
Tel: (510) 486-5268
Fax: (510) 486-4773
Email: alsuser@lbl.gov

For information on beamlines available to independent investigators, contact
Gary Krebs, ALS User Services Group Leader
Tel: (510) 486-7727
Fax: (510) 486-4102
Email: g_krebs@lbl.gov

4. SECRETS OF EXPRESS DELIVERY (Contact: GJGiangrasso@lbl.gov)

Users who are expecting to receive an express delivery package can ensure more timely delivery by utilizing ALS Receiving. Express couriers always deliver directly to the Berkeley Lab Shipping and Receiving Department, but once there, packages have to wait their turn for delivery by truck to the proper laboratory building. This can mean an afternoon delivery time or even delivery the next business day. If alerted, ALS Receiving can help prevent this delay. Users who know an express package is on its way to them should contact ALS Shipping Manager Gary Giangrasso (ext. 4494, Building 7, Room 100) with the name of the courier company and the name of the sender. Gary will ask the Berkeley Lab receiving clerks to notify him when the package arrives. When he gets the word, he will pick up the package directly from laboratory receiving and see that it reaches the user by 11:00 a.m.

5. INDEPENDENT INVESTIGATOR PROPOSAL SCORES ON THE WEB
(Contact: GFKrebs@lbl.gov)

Scores for the most recent round of Independent Investigator Proposals (IIPs) have now been posted on the World Wide Web at http://www-als.lbl.gov/als/quickguide/pspscores.html. This page shows the scores awarded by the Proposal Study Panel for all proposals at all beamlines and contains links to histograms for individual beamlines. By comparing the scores they received for their own submissions to the posted scores, investigators can gauge how close they came to being awarded beamtime. It is hoped that this will help them better determine whether or not to resubmit their proposals in the future.

6. ALAN JACKSON LEAVES ACCELERATOR PHYSICS GROUP

The Accelerator Physics Group bid adieu to leader Alan Jackson in early August. Jackson led the group from its inception, helping to establish the ALS's reputation for reliable operations and timely enhancements to accelerator capabilities. His involvement with the ALS accelerators goes all the way back to the drawing board. During construction of the ALS, Jackson was Deputy Director of the project and served as Accelerator Systems Group Leader. He guided the accelerators through construction, commissioning, and the generation of first light in 1993 and oversaw accelerator physics activities for six years thereafter. For the past two years, he has also served as Deputy Division Director for the Berkeley Lab Accelerator and Fusion Research Division (AFRD). He will now take charge of AFRD's Superconducting Magnet Program. This new job will keep at least some of his expertise at the ALS, as it will involve developing superconducting bend magnets for the storage ring. Jackson's Deputy, David Robin, will fill in as Acting Group Leader while a permanent replacement is sought. The transition is expected to be smooth, as Robin has already assumed many of the group's leadership functions, given Jackson's extra duties in AFRD.

7. WHO'S IN TOWN: A SAMPLING OF ALS USERS

To highlight the richness of our user community and help introduce recent arrivals, we offer this listing of some of the experimenters who will be collecting data during the next two weeks at the ALS.

Beamline 1.4.3: Miqin Zhang (Univ. of Washington) will continue studying the biocompatibility of microfabricated surfaces and medical devices. Upal Ghosh (Carnegie Mellon Univ.) will work on the identification, location, and nature of organic matter in sediments.

Beamline 6.3.2: Robert Schoenlein (Berkeley Lab) will work on femtosecond x-ray spectroscopy and the generation and measurement of femtosecond x-ray pulses. Ernie Glover (Berkeley Lab) will study femtosecond structural dynamics via optical gating.

Beamline 7.0.1: Jeffrey Terry (Los Alamos National Laboratory) will study itinerant and localized f-electrons in highly-correlated-electron materials. Ivan Schuller (Univ. of California, San Diego) will continue a spin-polarized photoemission study on the valence states of magnetite films. Also, Marjorie Olmstead (Univ. of Washington, Seattle) is measuring the electronic and structural properties of semiconductor/insulator interfaces by using angle-resolved photoemission.

Beamline 7.3.1.1: David Shuh (Berkeley Lab) will do photoemission electron microscope studies of technetium and rhenium sorption onto surfaces of iron metal, oxides, and sulfides.

Beamline 9.3.1: Heinz Frei (Berkeley Lab) will conduct x-ray absorption fine structure (XAFS) studies of bifunctional transition-metal molecular sieves for artificial photosynthesis.

Beamline 9.3.2: Brian Tonner (Univ. of Central Florida) will conduct a photoelectron diffraction study of clean and chemically modified mineralogical surfaces.

8. OPERATIONS UPDATE
(Contact: RMMiller@lbl.gov)

Beam reliability for user shifts was 97.9% for August 16 to 29. All outages were of short duration.

Long-term and weekly operations schedules are available on the Web (http://www-als.lbl.gov/als/accelinfo.html). Requests for special operations use of the "scrubbing" shift should be sent to Bob Miller (RMMiller@lbl.gov, x4738) by 1:00 p.m. Friday. The Accelerator Status Hotline at (510) 486-6766 (ext. 6766 from Lab phones) features a recorded message giving up-to-date information on the operational status of the accelerator.