Sorting events that occur in endosomes are very important for cellular trafficking of proteins and membranes along the endocytic pathway. The shape of sorting endosomes plays a major role in their function. However, the molecular mechanisms that control the shape of sorting endosomes are far from being elucidated. Motors and cytoskeleton can influence the shape of membranes, especially of high curvature, by establishing tension on membranes. I propose to study the role of an actin cytoskeleton-motor, myosin 1b (Myo1b), a mammalian class I myosin, in regulating the morphology of sorting endosomes. Myo1b localizes to sorting endosomes and when overexpressed increases the formation of tubules in endosomes. I hypothesize that Myo1b induces the formation of membrane tubules by recruiting actin filaments that generates a pulling force on endosomal membranes. The aim of my project is to understand the molecular mechanisms involved in tubule formation mediated by Myo1b in sorting endosomes in mammalian cells. First, I will identify the mechanism by which Myo1b localizes to sorting endosomes. Second, I will establish whether Myo1b recruits actin filaments to sorting endosomes. Third, I will show that Myo1b is required for the formation of tubules in membranes of sorting endosomes. Fourth, I will investigate which sorting events on endosomes are dependent on Myo1b function. These aims employ molecular biology, biochemistry, cell biology and biophysics methodologies in a novel approach to study the shape of sorting endosomes. Ultimately, I wish to unravel the molecular mechanism whereby the Myo1b-dependent recruitment of actin to endosomes regulates endosomal morphology and endocytic trafficking.