Actin pushes in bizarre places

Actin pushes in bizarre places ewis Tilney has always had a gift for looking for biology in strange places. His publishing history, he says, reads like a “Rogue’s gallery” of plants, fungi, parasites, and plenty of unusual invertebrate creatures. “It’s just ridiculous, and nowadays I couldn’t get away with it.” His colleagues say it is part of his genius to look in unusual systems to answer fundamental biology questions, but Tilney shrugs off that idea. “I have the attention span of a five-year-old,” he says. “It’s always wonderful to start a new project that’s curious and interesting. Then something else attracts my attention and I move on—it’s a lack of commitment on my part.” So it’s no surprise that one of Tilney’s major contributions—proposing actin polymerization as a method of force generation within the cell—came through two landmark papers characterizing unorthodox systems: the acrosomal reaction in both starfish and sea cucumber sperm, and the cell-to-cell motility of the Listeria monocytogenes bacterium. In the late 1960s, cytoplasmic actin that was not bundled into a contractile unit with myosin was just beginning to be recognized. At a lecture at Woods Hole Oceanographic Institute, Tilney heard Jean Dan describe the amazing acrosomal reaction of starfish sperm, which in seven seconds shoots out a process that is 45 times the length of the sperm cell. In Dan’s fuzzy pictures, Tilney could just make out some intracellular filaments inside the process. Could cytoplasmic actin be responsible for this fantastic reaction ? Using a glutaraldehyde and osmium tetroxide “mixed fix” that stabilized actin filaments, Tilney and his colleagues captured stunning pictures of the acrosomal process reaction with a clear bundle of filaments filling the process (Tilney et al., 1973). They identified the major protein in the acrosomal process as actin both by binding myosin in situ and by SDS gel electrophoresis. Since 80% of the actin was a monomer before the reaction and appeared amorphous in the images, Tilney L