New insights into the mechanisms of cytomotive actin and tubulin filaments.

Dynamic, self-organizing filaments are responsible for long-range order in the cytoplasm of almost all cells. Actin-like and tubulin-like filaments evolved independently in prokaryotes but have converged in terms of many important properties. They grow, shrink, and move directionally within cells, using energy and information provided by nucleotide hydrolysis. In the case of microtubules and FtsZ filaments, bending is an essential part of their mechanisms. Both families assemble polar linear protofilaments, with highly conserved interfaces between successive subunits; the bonding at these longitudinal interfaces is nucleotide dependent. Better understanding of the mechanisms by which nucleotide hydrolysis affects the bonding between subunits in filaments, and other structural changes related to the nucleotide hydrolysis cycles, has emerged from recent X-ray crystallographic and electron microscopic structures, showing eukaryotic or prokaryotic protofilaments in various states. Detailed comparisons of the structures of related proteins from eubacteria, archaea, and eukaryotes are helping to illuminate the course of evolution.