Microtubule-based endoplasmic reticulum motility in Xenopus laevis: Activation of membrane-associated kinesin during development

The endoplasmic reticulum (ER) in animal cells uses microtubule motor prote ins to adopt and maintain its extended, reticular organization. Although th e orientation of microtubules in many somatic cell types predicts that the ER should move toward microtubule plus ends, motor-dependent ER motility re constituted in extracts of Xenopus Laevis eggs is exclusively a minus end-d irected, cytoplasmic dynein-driven process. We have used Xenopus egg, embry o, and somatic Xenopus tissue culture cell (XTC) extracts to study ER motil ity during embryonic development in Xenopus by video-enhanced differential interference contrast microscopy. Our results demonstrate that cytoplasmic dynein is the sole motor for microtubule-based ER motility throughout the e arly stages of development (up to at least the fifth embryonic interphase). When egg-derived ER membranes were incubated in somatic XTC cytosol, howev er, ER tubules moved in both directions along microtubules. Data from direc tionality assays suggest that plus end-directed ER tubule extensions contri bute similar to 19% of the total microtubule-based ER motility under these conditions. In XTC extracts, the rate of ER tubule extensions toward microt ubule plus ends is lower (similar to 0.4 mu m/s) than minus end-directed mo tility (similar to 1.3 mu m/s), and plus end-directed motility is eliminate d by a function-blocking anti-conventional kinesin heavy chain antibody (SU K4). In addition, we provide evidence that the initiation of plus end-direc ted ER motility in somatic cytosol is likely to occur via activation of mem brane-associated kinesin.