The molecular basis for bundle selectivity of myosin X

Abstract

Eukaryotic cells organize their contents through trafficking along cytoskeletal filaments. When presented with many apparently similar alternatives within the cortex, it is important to understand if and how myosin motors identify the few actin filaments that lead to their correct destinations. Recently we showed that myosin X, an actin-based motor that concentrates at the distal tips of filopodia, selects the fascin-actin bundle at the filopodial core for motility. Myosin X, while poorly processive on single actin filaments, takes long processive runs on actin filaments tightly bundled by fascin. Such a fascin bundle is the precise actin structure to which myosin X motors localize in vivo. Using single molecule optical trapping experiments we have determined the step size of this motor to be 17 nm, which is nearly half of the 36 nm pseudo-helical actin repeat required for motors to be processive on single actin filaments. These results indicate that straddling two filaments within a bundle stimulates this motor's function. Our initial model attributed this motility to the short lever arms of myosin X, consisting of 3 IQ repeats rather then the six found in the processive myosin V. To test this, chimeras were constructed where the heads, the IQ domains and the post IQ sequence (containing the coiled-coil dimerization domains) of Myosin V and Myosin X were used to create six combinatorial constructs. Single molecule fluorescence studies of these constructs revealed that the post IQ region and not the short lever arm of this motor is the main contributor to its unique selectivity. This result provides remarkable insight into the ability of nature to fine-tune myosin motors to serve their specific functions in the cell.