Intracellular vesicle transport pathways and localization of selected Rabs. The biosynthetic pathway transports proteins from the endoplasmic reticulum (ER) through the Golgi complex to the cell surface. In the trans-Golgi network (TGN), molecules can enter either constitutive secretory vesicles (CV) or regulated secretory granules/vesicles (RV). In specialized cells melanosomes (M) are a lysosome-related compartment that moves within the cell in an actin- and myosin-dependent manner, generating pigmentation. Material internalized from outside the cell reaches the early endosomes (EE) first and can be recycled back to the surface, either directly or via a perinuclear recycling endosome (RE) compartment, or transported to late endosomes (LE) and lysosomes. The biosynthetic and endocytic circuits (arrows) exchange material at the level of the Golgi apparatus and the endosomal elements. The localization of selected mammalian Rab proteins in the membrane compartments participating in these transport processes is indicated.