We show that the Golgi complex can directly stimulate microtubule nucleation in vivo and in vitro and thus behaves as a potent microtubule-organizing organelle in interphase cells. With the use of nocodazole wash-out experiments in hepatic cells, we found that the occurrence of noncentrosomal, early stabilized microtubules is highly correlated with the subcellular localization of Golgi membranes. With the use of in vitro reconstituted microtubule assembly systems with or without cytosol, we also found that, in contrast to centrosomally attached microtubules, the distal ends of Golgi-attached microtubules are remotely stabilized in a way that requires additional cytosolic component (s). Finally, we demonstrate that Golgi-based microtubule nucleation is direct and involves a subset of ␥-tubulin bound to the cytoplasmic face of the organelle. INTRODUCTIONIn interphase cells, the microtubule (MT) network plays a major role in membrane dynamics and organelle localization. In this context, the interaction between the Golgi complex and the MT network has been extensively studied. The Golgi apparatus colocalizes with the minus ends of MTs, which are usually associated with the centrosome (for review see Kreis et al., 1997;Thyberg and Moskalewski, 1999). This localization actually results from an equilibrium between two contradictory movements that have been unraveled with the use of MT-depolymerizing drugs and probably occur on MT subpopulations with different dynamics. The dispersal of Golgi elements occurs along stable MTs after depolymerization of the most labile MT population (Minin, 1997), whereas their reclustering involves newly assembled MTs (Ho et al., 1989). These contradictory movements of Golgi elements are mediated by distinct molecular motors. Consistent with earlier findings by Feiguin et al. (1994), who found that the expression of kinesin antisense oligonucleotide rendered the Golgi apparatus more compact, microinjection of antikinesin antibodies inhibited Golgi dispersion along stable, nocodazole-resistant MTs (Minin, 1997). Conversely, the central localization of the Golgi apparatus involves cytoplasmic dynein (Corthésy-Theulaz et al., 1992;Fath et al., 1994;Burkhardt et al., 1997;Harada et al., 1998). It is also worth noting that, in addition to the kinesin-mediated disruption of the central Golgi complex during MT depolymerization, the dispersion of Golgi elements also relies on the reconstitution of mini Golgi stacks at endoplasmic reticulum (ER) exit sites (Cole et al., 1996;Storrie et al., 1998). Such data on the scattering and the reclustering of the Golgi complex have led to the view that Golgi membranes undergo an intimate relationship with MTs and especially with a population of nocodazole-resistant, stable MTs. Stable MTs are characterized by the occurrence of posttranslationally modified tubulin-especially detyrosinated and acetylated tubulin (for review, see MacRae, 1997), which is thought to accumulate in stable MTs because of their longer half-lives, but does not influence MT stability i...

No abstract