Radial extension of macrophage tubular lysosomes supported by kinesin

The centrifugal elongation of membranes to form extended tubular structures is a widespread form of intracellular organelle movement. Tubular lysosomes and the endoplasmic reticulum, for example, undergo such extension in association with microtubules, and this process has been mimicked in vitro by combining purified microtubules with isolated membranes and the mechanochemical ATPase kinesin. This, along with evidence that kinesin is associated with the endoplasmic reticulum, has led to the suggestion that kinesin provides the motive force for the formation and maintenance of elongated tubulovesicular structures in cells. We have addressed this hypothesis in murine macrophages, which have prominent tubular lysosomes whose form depends on the integrity of microtubules. Here we report that two antikinesin antibodies which disrupt in vitro motility will each cause centripetal collapse of the array of tubular lysosomes when scrape-loaded into macrophages. To our knowledge this provides the first in vivo evidence that kinesin is responsible for extension of tubulovesicular structures along microtubules.