MOVEMENT OF MEMBRANE TUBULES ALONG MICROTUBULES IN-VITRO - EVIDENCE FOR SPECIALIZED SITES OF MOTOR ATTACHMENT

We have studied the microtubule-dependent formation of tubular membran e networks in vitro, using a heterologous system composed of Xenopus e gg cytosol combined with rat liver membrane fractions enriched in eith er Golgi stacks or rough endoplasmic reticulum. The first step in memb rane network construction involves the extension of membrane tubules a long microtubules by the action of microtubule-based motor proteins. W e have observed for both membrane fractions that 80-95% of moving tubu le tips possess a distinct globular domain. These structures do not fo rm simply as a consequence of motor protein activity, but are stable d omains that appear to be enriched in active microtubule motors. Negati ve stain electron microscopy reveals that the motile globular domains associated with the RER networks are generally smaller than those obse rved in networks derived from a crude Golgi stack fraction. The globul ar domains from the Golgi fraction are often packed with very low dens ity lipoprotein particles (the major secretory product of hepatocytes) and albumin, which suggests that motor proteins may be specifically e nriched in organelle regions where proteins for export are accumulated . These data raise the possibility that the concentration of active mo tor proteins into specialised membrane domains may be an important fea ture of the secretory pathway.