For instance, rab4 acts downstream of rab5 in endosomal transport, and activated rab5 controls the spatial and temporal properties of rab4 activation by regulating its GTPase exchange factor (Miaczynska et al., 2004). Multiple rabs in a pathway associate and

dissociate from endosomes in a regulated, sequential manner, and not surprisingly, many of the regulatory families interact and regulate one another, thereby forming interconnected regulatory networks. The simultaneous action of these mechanisms contributes to compartment identity and ensures vectorial transport. Owing to the advancement in imaging techniques, especially high-resolution live-cell imaging, it is now clear that the endosomal system is very dynamic and more complex than previously anticipated (Kirchhausen, 2009, Lakadamyali et al., 2006, Mattheyses selleck chemical et al., 2011, Sönnichsen et al., 2000 and Zoncu et al., 2009). One long-standing question in the field of membrane trafficking is the stability of endosomal compartments in time and space. For the biosynthetic system, a “stable compartment” model has been favored in which enduring ER and Golgi compartments are connected via mobile small vesicular carriers Protein Tyrosine Kinase inhibitor that deliver and remove cargos

(Pfeffer, 2010 and Polishchuk et al., 2003). Membrane-associated regulators required for directed fusion (such as SNAREs) are then temporarily found in the “incorrect” compartment after fusion and need to be recycled back to their compartment of origin. In the endosomal system, evidence supporting a “maturation” model has emerged in which earlier compartments in the pathway recruit successively new regulators (such as rabs and lipid-modifying enzymes), which change the compartment identity over time (Poteryaev et al., 2010 and Zoncu et al., 2009). Live-cell imaging of

endosomal compartments labeled with L-NAME HCl different endosomal regulators showed that pre-early endosomal compartments (APPL-positive) convert to early endosomes (EEA1-positive) (Zoncu et al., 2009), by shedding APPL and recruiting EEA1 to the same pre-existing endosome. Little is known about how the conversion of one compartment to another is achieved. For the switch from the APPL-positive preEE to the EEA1-positive EE, rab5 activity and accumulation of a different phosphoinositide species, PI-3P, are required (Zoncu et al., 2009). Rab 5 is at some point rapidly removed from the early endosome and replaced by rab7 (Rink et al., 2005) in a coordinated “rab conversion” event. Similar rab conversions also occur on other compartments as they mature. Rab7-containing endosomes will mature toward the late endosomal fate by recruiting additional machinery, such as ESCRT complex (Henne et al., 2011). How rab5 converts to a rab7-positive LE is not known, but rab activity and phosphoinositides probably play a role here as well.