DANCING WITH THE ELEPHANT: STRUCTURE AND ASSEMBLY OF THE FIBER-FORMING PROTEIN PILIN. John A. Tainer, Hans E. Parge, Katrina T. Forest, and Elizabeth D. Getzoff. Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037

Attempting the three-dimensional description of protein fiber assemblies calls to mind the problem embodied in the Hindu fable about six erudite but blind men who came to "see" the elephant. Finding distinct elephant regions prompted their disparate comparisons to a wall, a spear, a snake, a tree, a fan, and a rope, so "...each was partly in the right and all were in the wrong." Sir Lawrence Bragg underscored the difficulty of applying crystallography to visualize fiber assemblies by dividing proteins into two broad classes: the globular proteins, which often exist as individual molecules and form excellent crystals, and the fiber-forming proteins that aggregate but do not crystallize due to the complex ways individual molecules aggregate (Bragg, 1975). Yet, without crystallographic structures, we risk the blind man's confusion in attempting the integration of the genetic, biochemical, and biological data for these critical and often multi-functional cellular assemblies.

Type IV pili are long, multi-functional fibers involved in the attachment, mobility, DNA transformation, and infectivity of many bacterial pathogens. Moreover, these intrinsically flexible pili bend, extend, and retract suggesting that the elephant we wish to visualize is not standing still but dancing. We isolated pilus fibers, disassembled them with high pH and n-octyl-[[beta]]-D-glucopyranoside and obtained diffraction quality crystals by adding 1,2,3-heptanetriol. The 2.6 Å pilin structure reveals a novel ladle-shaped [[alpha]]-[[beta]] roll fold with an extended [[alpha]]-helical spine. The pilin crystal structure combined with results from electron and force microscopy, fiber diffraction, and antibody binding suggests a testable assembly model and a means of dancing with the elephant, so that we approach a detailed understanding of pili function and assembly. Questions that can now be addressed include the structural basis for the high mechanical stability required for a fiber whose length (40,000 Å) is over 600 times its diameter (60 Å), and how these stealth fibers can undergo extreme sequence variation to escape the host immune response while maintaining their assembly and function.

Bragg, L. (1975) The development of x-ray analysis. Dover, NY.