FLUORESCENCE INTERFERENCE CONTRAST MICROSCOPY, A NEW TOOL TO STUDY COLLECTIVE MOTOR DYNAMICS

Teams of molecular motors called myosins carry out intracellular transport and contract the actin cytoskeleton. To fully understand the behavior of multi-myosin ensembles we need to know the properties of individual myosins and the mode of interaction between them. Current models of the interactions within the myosin complex treat the actin filament as a stiff rod, not contributing to the regulation of collective myosin dynamics. Here, I present data suggesting that force transduction through the actin filament is an important element of interaction within myosin-6 ensembles in vitro. Multiple myosin-6s coordinate their steps if they are separated by a short (and therefore high-force bearing) segment of actin. The measurements were performed using Fluorescence Interference Contrast Microscopy (FLIC). In FLIC, the fluorescence intensity is proportional to the height of a fluorophore above a reflective surface. FLIC allows for the detection of small changes in a height of fluorescently labeled actin. Using FLIC, I was able to assign the positions of myosins in a gliding filament assay geometry and to measure the time each of them spends bound to actin (the attachment time). I show that myosin-6 holds actin about 10 nm above the surface. However, due to asynchronous myosin stepping, frequent buckles up to about 70 nm high appear. The buckle lifetime decreases as the distance between the myosin-6s is reduced, a sign of inter-motor coordination. Similar measurements were performed for myosin-5 and NMIIB, showing that the FLIC assay is a robust and versatile technique allowing for ensemble measurements with single molecule insight.