It is more likely that molecules such as Rab5 and Rabaptin5 provide an additional layer of regulation on the vesicle transport system, integrating the temporal regulation with the specificity of membrane-membrane recognition. These two features make it possible for membrane trafficking to occur at the correct rate and between the appropriate compartments. Coated vesicles transport materials and information to cellular destinations, but they need surface markers to identify the vesicles according to origin, cargo and destination, as well as to time the process. Rab proteins play an essential role in this machinery. In simple terms, Rab proteins act as molecular switches and timers that regulate when and where vesicles are transported.
Every organelle has at least one Rab for each transport step.
The amino acid sequences of a Rab's carboxyl-terminal tail carries a code that determines its destination. Different Rab proteins switch between active and inactive forms at different rates. The velocity of the transport cycle is determined by the frequency of protein oscillattion between active and inactive form. The Rab5 switch, for example, is relatively fast and perfectly suited for the short journey from the plasma membrane to the early endosome - the main sorting station on the endocytic pathway into the cell. Rab works in a cycle.