ARTICLES Myosin 1b promotes the formation of post-Golgi carriers by regulating actin assembly and membrane remodelling at the trans -Golgi network Claudia G. Almeida 1,2 , Ayako Yamada 1,3 , Danièle Tenza 1,4,5 , Daniel Louvard 1,2 , Graça Raposo 1,4,5 and Evelyne Coudrier 1,2,6 The function of organelles is intimately associated with rapid changes in membrane shape. By exerting force on membranes, the cytoskeleton and its associated motors have an important role in membrane remodelling. Actin and myosin 1 have been implicated in the invagination of the plasma membrane during endocytosis. However, whether myosin 1 and actin contribute to the membrane deformation that gives rise to the formation of post-Golgi carriers is unknown. Here we report that myosin 1b regulates the actin-dependent post-Golgi traffic of cargo, generates force that controls the assembly of F-actin foci and, together with the actin cytoskeleton, promotes the formation of tubules at the TGN. Our results provide evidence that actin and myosin 1 regulate organelle shape and uncover an important function for myosin 1b in the initiation of post-Golgi carrier formation by regulating actin assembly and remodelling TGN membranes. The trans-Golgi network (TGN) is one of the main protein sorting stations of the cell at the crossroads of the exocytic and endocytic pathways. This organelle is composed of a complex tubular network that emanates from the trans-Golgi cisternae, and generates pleiomorphic carriers targeted to different destinations. Its shape is intimately associated with its function. Marked changes in its membrane shape underlie the exit of cargos. This exit involves concentration of cargo in membrane domains of the TGN, membrane deformation or budding, elongation of tubular-carrier precursors and scission leading to the formation of the post-Golgi carriers. Post-Golgi carriers are then transported through the cytosol along microtubules. There are various possible mechanisms that can regulate membrane deformation, which include changes in lipid composition, oligomerization of scaffolding proteins or insertion of transmembrane proteins, that through their shape or clustering can bend membranes 1 . The cytoskeleton and its motors, which can change the mechanical properties of membranes, also play a major role in membrane deformation 1 . Growing evidence indicates that the actin cytoskeleton plays a role in membrane trafficking at the TGN, and numerous proteins related to the actin-based system have been now localized to this organelle 2–5 . However the role of actin dynamics at an early stage of post-Golgi carrier biogenesis, such as membrane deformation, is much 1 Institut Curie, Centre de Recherche, Paris, F-75248, France. 2 Morphogenesis and Cell Signalization CNRS, UMR144, Paris, F-75248, France. 3 Cell and Tissue Imaging Facility, CNRS UMR 144, Paris F-75248, France. 4 Membrane and Cell Functions, CNRS UMR 168, Paris F-75248, France. 5 Structure and Membrane Compartments CNRS, UMR144, Paris, F-75248, France. 6 Correspondence should be addressed to E.C. (e-mail: evelyne.coudrier@curie.fr) Received 3 June 2010; accepted 18 April 2011; published online 12 June 2011; DOI: 10.1038/ncb2262 less documented. Two myosins, myosin 6 and myosin 2, contribute to post-Golgi traffic 6–8 and recent evidence indicates that myosin 2 is required for scission 9 , but the possible role of a myosin in an earlier step of the post-Golgi carrier biogenesis remains to be established. In this report we show that a myosin 1 (myosin 1b) and the actin cytoskeleton promote the formation of tubular-carrier precursors. Together, our data demonstrate that myosin 1b is a key regulator of membrane remodelling at the TGN by controlling filamentous actin (F-actin) assembly in this region. RESULTS A pool of myosin 1b is associated with F-actin foci and CI-mannose-6-phosphate receptor in the perinuclear region Myosin 1b (Myo1b) is involved in the traffic of cargo along the endocytic pathway 10–12 . It has been localized at the plasma membrane in regions enriched for actin filaments and at early endosomes, multivesicular endosomes and lysosomes 11–13 . A pool of Myo1b was also found to co-fractionate with Golgi markers and detected by immunofluorescence in the perinuclear region of mouse hep- atoma cells 11,14 . In agreement with these observations, we observed an accumulation of Myo1b in the perinuclear region of HeLa cells (Fig. 1a). This pool co-localizes partially with TGN46 and CI-mannose-6-phophate receptor (MPR) that carries cargos from NATURE CELL BIOLOGY VOLUME 13 | NUMBER 7 | JULY 2011 779