Lytic granules from cytotoxic T cells exhibit kinesin-dependent motility on microtubules in vitro

One major mechanism of cell-mediated cytolysis is the polarized secretion of lytic granules, a process which is highly dependent on microtubules. We isolated lytic granules from murine cytotoxic T cells and tested their ability to bind to and move along microtubules in vitro. In the presence of a motor-containing supernatant, the granules bound to the microtubules and moved along them at an average maximal rate of 1 μm/second. Virtually every granule could bind to microtubules, and about half translocated within a few seconds of binding. Motility required exogenous cytosolic motors, hydrolyzable nucleotides, and an intact granule membrane. Although the motor preparation used to support granule movement contains both plus- and minus-enddirected motor proteins, granule movement was strongly biased toward microtubule plus-ends. Inactivation of cytoplasmic dynein had little effect on granule binding and movement, but immuno-depletion of kinesin from the motor preparation inhibited granule binding by 50%. These results indicate that most granule movement in this assay is mediated by kinesin. The speed and direction of granule movement in vitro are sufficient to account for the release of lytic granules in the intact T cell. This model system should be valuable for studying the interactions of secretory granules with microtubules, and for identifying the regulatory factors involved.