Esther S. Axelrod Bullitt, Ph.D.

Phone: (617) 638-5037 • Fax: (617) 638-4041
e-mail: bullitt@bu.edu
address: click here

Research

Protein Structure Facilitates Function:

Using Electron Microscopy and Quantitative Image Analysis to see how Macromolecular Assemblies Work

Structural studies of biological macromolecular assemblies are providing an understanding of cellular function. In our laboratory, we utilize electron microscopy and image reconstruction to investigate questions about how adhesion pili aid pathogenic bacterial survival under harsh physiological conditions.

Bacterial Adhesion

	Fibers on the surface of bacteria that cause urinary tract infections (red) can overextend like a toy Slinky(left), whereas homologous proteins on bacteria that cause pneumonia, ear infections, and meningitis assemble into a 3-stranded fiber (blue). The 3-stranded fiber is a stronger structure that is resistant to damage by coughs and sneezes.

Worldwide, approximately 150 million people are diagnosed with urinary tract infections (UTIs) each year, costing over $6 billion. In the U.S., UTIs are responsible for more than 7 million doctor's office visits, over 1 million hospital admissions, and at a cost of approximately $1 billion per year.

As more bacteria become resistant to antibiotics, it is essential to develop novel approaches for prevention and treatment of infection. Clues to how bacteria bind, and how they remain attached while the host is trying to remove them, are found by examining the architecture of bacterial adhesion pili. These pili are hair-like surface fibers that extend from the bacteria's surface, and attach these bacteria to their target cells.

The urinary tract may seem an especially harsh environment, but there are strains of Escherichia coli (E. coli) designed specifically for binding to surface-exposed cells there. Bacterial infection is dependent on these pili; without them the bacteria are washed away, presumably by the fluid flow of urine. Another bacterium, Haemophilus influenzae, has specialized pili that are adapted to binding to cells in the nose, throat and lungs, causing pneumonia in the elderly and meningitis in young children.

Detailed structural information on pili is needed; understanding how pili are built will help us design effective anti-bacterial vaccines and drugs. Using electron microscopy and computer image analysis, our research investigates the structure of bacterial pili. The power of these techniques lies in the greater detail and improved reliability of results obtained by averaging data from many pili. Our initial research has shown that each pilus is a long thin helix, and that in Escherichia coli even damaged pili may maintain contact between the bacterium and its host, allowing infection to persist. Current studies are investigating both intact and damaged pili, to determine whether repair occurs. In the future, drugs may be designed to damage pili first, and subsequently destroy them.

Structural studies of pili can teach us about bacterial binding, and lead to clues for designing vaccines and drugs that prevent bacterial binding and, therefore, prevent infections that cause disease.

More Details

Poliovirus Polymerase Oligomerization

Schematic model of lattice formed by poliovirus 3Dpol (pdb 1rdr) superposed on an electron micrograph of negatively stained helical tubes, two-dimensional lattice, and twisted sheets of 3Dpol.

Lattice formation by the positive-strand poliovirus polymerase may increase the reaction rate of replication by concentrating substrate and enzyme onto a two-dimensional surface, the vesicular membrane.

The poliovirus polymerase, 3Dpol, is an RNA-dependent RNA polymerase. Its structure is similar to almost all polymerases, and can be described as a right hand. We are investigating the role of oligomerization in replication, using electron microscopy and image processing of tubes and sheets formed in vitro. The crystal structure in the image below was solved in Steve Schultz's lab, pdb 1rdr.

Lyle, J.M., E. Bullitt , K. Bienz, and K. Kirkegaard (2002). Visualization and functional lattices of RNA-dependent RNA polymerase lattices . Science 296:2218-2222

Hansen, J.L., A.M. Long, S.C. Schultz (1997). Structure 5:1109

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Publications

Mu XQ, Egelman EH, Bullitt E. Structure and Function of Hib Pili from Haemophilus influenzae Type b. J Bacteriol. 2002 Sep;184(17):4868-74.

Lyle JM, Bullitt E, Bienz K, Kirkegaard K. Visualization and functional analysis of RNA-dependent RNA polymerase lattices. Science. 2002 Jun 21;296(5576):2218-22.

Saulino ET, Bullitt E, Hultgren SJ. Snapshots of usher-mediated protein secretion and ordered pilus assembly Proc Natl Acad Sci USA 2000 Aug 1;97(16):9240-9245.

Bullitt E, Makowski L. Bacterial adhesion pili are heterologous assemblies of similar subunits. Biophys J 1998 Jan;74(1):623-32.

Bullitt E, Rout MP, Kilmartin JV, Akey CW. The yeast spindle pole body is assembled around a central crystal of Spc42p. Cell 1997 Jun 27;89(7):1077-86.

Bullitt E, Jones CH, Striker R, Soto G, Jacob-Dubuisson F, Pinkner J, Wick MJ, Makowski L, Hultgren SJ. Development of pilus organelle subassemblies in vitro depends on chaperone uncapping of a beta zipper. Proc Natl Acad Sci U S A 1996 Nov 12;93(23):12890-5.

St Geme JW, Pinkner JS 3rd, Krasan GP, Heuser J, Bullitt E, Smith AL, Hultgren SJ. Haemophilus influenzae pili are composite structures assembled via the HifB chaperone. Proc Natl Acad Sci USA 1996 Oct 15;93(21):11913-8.

Bullitt E, Makowski L. Structural polymorphism of bacterial adhesion pili. Nature 1995 Jan 12;373(6510):164-7.

Contact Us

Department of Physiology and Biophysics
Boston University School of Medicine
700 Albany Street
Boston MA 02118-2526

Phone: (617) 638-5037
Fax: (617) 638-4041
e-mail: bullitt@bu.edu