Members of the Rho family of small GTPases control cell adhesion and motili
ty through dynamic regulation of the actin cytoskeleton. Although twelve fa
mily members have been identified, only three of these RhoA, Pac and Cdc42
- have been studied in detail. RhoA regulates the formation of focal adhesi
ons and the bundling of actin filaments into stress fibres. It is also invo
lved in other cell signalling pathways including the regulation of gene exp
ression and the generation of lipid second messengers [1,2]. RhoA is very c
losely related to two other small GTPases about which much less is known: R
hoB and RhoC (which are approximately 83% identical). Perhaps the most intr
iguing of these is RhoB. RhoA is largely cytosolic but translocates to the
plasma membrane on activation. RhoB, however, is entirely localised to the
cytosolic face of endocytic vesicles [3,4], This suggests a potential role
for RhoB in regulating endocytic traffic; however, no evidence has been pre
sented to support this. RhoA has been shown to act at the plasma membrane t
o regulate the clathrin mediated internalisation of transferrin receptor [5
] and of the muscarinic acetylcholine receptor [6]. We have recently demons
trated that RhoB binds the RhoA effector, PRK1 and targets it to the endoso
mal compartment [7]. We show here that RhoB acts through PRK1 to regulate t
he kinetics of epidermal growth factor receptor traffic.