Rab-GDI presents functional Rab9 to the intracellular transport machinery and contributes selectivity to Rab9 membrane recruitment

Quantitative analysis of the interactions between prenyl Rab9, GDP dissociation inhibitor-alpha, and guanine nucleotides

Rab9 is a Ras-like GTPase required for the transport of mannose 6-phosphate receptors between late endosomes and the trans Golgi network. Rab9 occurs in the cytosol as a complex with GDP dissociation inhibitor (GDI), which we have shown delivers prenyl Rab9 to late endosomes in a functional form. We report here basal rate constants for guanine nucleotide dissociation and GTP hydrolysis for prenyl Rab9. Both rate constants were influenced in part by the hydrophobic environment of the prenyl group. Guanine nucleotide dissociation and GTP hydrolysis rates were lower in the presence of lipid; detergent stimulated intrinsic nucleotide exchange. GDI-alpha inhibited GDP dissociation from prenyl Rab9 by 2.4-fold. GDI-alpha associated with prenyl Rab9 with a KD of 60 nM in 0.1% Lubrol and 23 nM in 0.02% Lubrol. In 0.1% Lubrol, GDI-alpha inhibited GDP dissociation half maximally at 72 +/- 18 nM, consistent with the KD determinations. These data suggest that GDI-alpha associates with prenyl Rab9 with a KD of < or = 23 nM under physiological conditions. Finally, a previously uncharacterized minor form of GDI-alpha inhibited GDP dissociation from prenyl Rab9 by 1.9-fold and bound prenyl Rab9 with a KD of 67 nM in 0.1% Lubrol.

PDGF stimulates an increase in GTP-Rac via activation of phosphoinositide 3-kinase

BACKGROUND

Phosphoinositide 3-kinases (PI 3-kinases) are thought to play an important role in coordinating the responses elicited by a variety of growth factors, oncogene products and inflammatory stimuli. These responses include activation of membrane ruffling, chemotaxis, glucose transport, superoxide production, neurite outgrowth and pp70 S6 kinase. Some of these responses are also known to be regulated by Rac, a small GTP-binding protein related to Ras. Neither the transducing elements upstream of Rac, nor those downstream of PI 3-kinase, have been defined.

RESULTS

We show here that platelet-derived growth factor (PDGF) can stimulate an increase in the level of GTP-Rac by at least two distinct mechanisms: firstly, by increased guanine nucleotide exchange; and secondly, by inhibition of a Rac GTPase activity. The first of these mechanisms is essential for the activation of Rac, and we show that it is dependent upon PDGR-stimulated synthesis of phosphatidylinositol (3,4,5)-trisphosphate.

CONCLUSIONS

These results suggest that Rac activation lies downstream of PI 3-kinase activation on a PDGF-stimulated signalling pathway. Furthermore, as Rac has been implicated in at least two diverse cellular responses that are also though to require activation of PI 3-kinase--a reorganization of the actin cytoskeleton known as membrane ruffling and the neutrophil oxidative burst--these results suggest that Rac may be a major effector protein for the PI 3-kinase signalling pathway in many cell types.

Rab GTPases: master regulators of membrane trafficking

Rab GTPases are thought to be likely to catalyze the accurate association of pairs of targeting molecules located on the surfaces of transport vesicles with their corresponding membrane acceptors. Advances during the past year have solidified our understanding of the mechanisms by which Rab proteins are recruited onto nascent transport vesicles and retrieved from their fusion targets. Functional analyses of Rab proteins in living cells have led to the surprising observation that vesicles do not seem to form if the appropriate Rab protein, in its GTP-bound conformation, is not present.