Virtuelle (Nano) Welten: PDB Molecule of the Month 19: Actin

Controlled Growth

Of course, cells cannot have actin filaments growing uncontrollably all over the cytoplasm. The poison phalloidin, from the death cap mushroom, demonstrates what would happen. It promotes the growth of actin and ultimately clogs the cell with rigid actin filaments, causing fatal liver and kidney damage in unwary mushroom lovers. In cells, a variety of actin-severing proteins control the growth of actin, ensuring that the filaments grow only when needed. Two of these actin watchdogs are shown here, with actin in blue, ATP in green, and the actin-binding protein in red and orange. Gelsolin, on the left from PDB entry 1yvn, breaks actin filaments into short lengths when the level of calcium rises. Then, it remains bound to the end, blocking additional growth. Profilin, on the right from PDB entry 1hlu, binds to free actin and keeps it from adding to filaments, also inhibiting growth. Both bind to the actin monomer at a similar location, blocking part of the site that binds to neighboring actin molecules in a filament.