Fig.1: Depending on environmental conditions, both the Mycobacterium tuberculosis and Escheria coli cell division protein FtsZ can assemble into a diverse array of two- and three-dimensional superstructures, including rings (top left), helices (bottom left), planar waves (bottom right) and donut-like toroids (top right) (scale bars 500 nm, except top right, 200 nm).
© 2010 D. Popp
Cells rely on extensive networks of protein fibers to help maintain their proper form and function. For species ranging from bacteria to humans, this ‘cytoskeleton’ is integrally involved in diverse processes including movement, cell division and transport of molecular cargoes.
However, scientists are still far from a complete understanding of these various filaments. “Genetic screening has recently identified over 40 new cytoskeletal components in bacteria with unknown functions and structures,” explains David Popp of the A*STAR Institute of Molecular and Cell Biology, Singapore. Now, in collaboration with Japanese researchers from the ERATO ‘Actin Filament Dynamics’ project, Popp’s group has published several studies that offer important insights into several of these enigmatic bacterial proteins.
The filament-forming protein F-actin is remarkably well-conserved across eukaryotic species. “From chicken to human—an evolutionary distance of some 300 million years—not a single residue has changed in skeletal muscle actin, and the filament structure has been highly conserved,” says Popp. However, he and his co-workers were surprised to observe striking diversity among bacterial actins.
For example, they found that Bacillus subtilis AlfA polymerizes at a considerably faster rate than other actins, forming filaments with a highly distinctive helical arrangement1. Likewise, their analysis of Staphylococcus aureus pSK41 ParM revealed that although this protein is a known homolog of actin relative R1 ParM, it assembles into a markedly different structure that more closely resembles filaments formed by a distantly related archaeal protein2. Collectively, these findings suggest that despite the broad structural similarities that unite actin proteins as a common family, individual proteins can exhibit highly divergent polymerization characteristics that may make them uniquely suited for the cellular processes in which they participate.
In parallel, Popp and his co-workers investigated the dynamic behavior of a different type of cytoskeletal factor, the microtubule-like cell division protein FtsZ, from the bacterium responsible for tuberculosis3. Depending on the environmental conditions, this protein can assemble into a diverse variety of two- and three-dimensional structures (Fig. 1). They found strong evidence that this conformation depends heavily on ‘molecular crowding’ effects arising from elevated protein density in different compartments of the cell.
All three of these proteins execute essential cellular functions, and Popp concludes that further progress in this area may yield far-reaching benefits in a range of research areas. “We expect these investigations to give answers to many fundamental questions about the cytoskeleton,” he says, “and they will also contribute to the design of better manmade biopolymers and more efficient design of drugs targeting the bacterial cytoskeleton.”
The A*STAR-affiliated authors in this highlight are from the Institute of Molecular and Cell Biology.
