Step 4

Rab5, like all small GTPases, acts as a molecular switch in signaling pathways to regulate the functions of other proteins and is controlled through an activation and deactivation cycle regulated by GTP-binding proteins. Rab5 is activated and able to carry out its function when bound to guanosine triphosphate (GTP) and inactive when bound to guanosine diphosphate (GDP). Activation of Rab5 is facilitated by a 150kDa guanine nucleotide exchange factor (GEF) discovered in the lab called Rab5 activating protein 6 (RAP6). All small GTPases are deactivated by GTPase activating proteins (GAPs) and are cyclically switched on and off by the activity of GEFs and GAPS, respectively. In studying RAP6 it has become evident that this protein contains a nuclear localization signal that links Rab5 to nuclear signal transduction.

In 2006, researchers from Florida International University discovered a novel GEF (activating protein) for Rab5, named Rab5 activating protein 6 (or RAP6). My investigation revolved around examining the properties and function of RAP6 in the cell. Thus far, it is understood that there are three distinct regions within this protein with different characteristics. These domains are known as Ras GAP domain (RGD), Pleckstrin Homology domain (PH), and Rab5 GEF domain (the novel finding that RAP6 activated Rab5 through this domain). In the figure above, these domains are outlined on the RAP6 gene and the corresponding amino acid sequences are color-coded. The RGD domain seems to have Ras GAP activity, meaning it inactivates another signaling protein called Ras. The misregulation of this protein is often implicated in cancer. Then, the PH domain is seen in other models to play a role in cell signaling and trafficking (regulated transportation of material within the cell), and finally, the Rab5 GEF domain is known to activate Rab5 in early endosome fusion events. Currently, there is ongoing work on further characterizing the function of RAP6 in diseased cells.