User:Barberm01/prex1

P-Rex1 ( P hosphatidylinositol (3,4,5)-triphosphate-dependent R ac ex changer), is an enzyme (~185 kDa) involved in intracellular signalling networks. P-Rex1 is highly expressed in leukocytes and the brain and activates the small Guanosine triphosphate (GTP)-binding protein (G protein) Rac. P-Rex1 controls leukocyte function and neuronal morphology, contributes to the regulation of motor coordination, and promotes cancer metastasis. P-Rex1 was the first member of the P-Rex enzyme family to be described and belongs to the Dbl family of Guanine nucleotide exchange factors (GEFs). 

Discovery
P-Rex1 was purified from pig leukocyte cytosol in a screen for proteins mediating phosphatidylinositol (3,4,5)-triphosphate (PIP3)-dependent activation of the Rho-family G protein (GTPase) Rac1. A cytosolic fraction corresponding to the major peak of PIP3-sensitive Rac-GEF activity was purified to yield a preparation that contained a single pure protein which was identified using MALDI-TOF and N-terminal sequencing and named P-Rex1. The protein contains tandem DH/PH domains, typical of Dbl family GEFs, two DEP and two PDZ domains, and significant sequence similarity over its C-terminal half to Inositol Polyphosphate 4-Phosphatases.

Function and Activation
P-Rex1 contributes to cellular signalling events by activating the Rho-family G proteins Rac1 and Rac2 (with a preference for Rac2 in vivo.  In their resting state G proteins are bound to Guanosine diphosphate (GDP) and their activation requires the dissociation of GDP and binding of GTP.  P-Rex1 is assumed to act as a classical Dbl family GEF, which activates small G proteins by stabilising the nucleotide free form, allowing excess free cytosolic GTP to bind.  P-Rex1 is directly activated by the lipid second messenger PIP3, which is generated by phosphoinositide 3-kinase (PI3K), via it's PH domain, and was the first Rho GEF shown to be directly stimulated by the Gβγ subunits of heterotrimeric G proteins.   P-Rex1 is maximally stimulated by Gβγ heterodimers containing Gβ1-4 complexed with γ2 while the Gα subunits Gs, Gi, Gq, G12, and G13 are unable to activate P-Rex1. The cAMP-dependent kinase Protein kinase A (PKA) phosphorylates P-Rex1 and negatively regulates it's PIP3- and Gβγ-dependent GEF activity both in vitro and in vivo. Truncation of protein domains C-terminal to the catalytic DH/PH tandem leads to increased basal and stimulated GEF activity, suggesting some level of interaction between these domains and the catalytic core in the basal state of the enzyme.

P-Rex1 exists mainly in the cytosol but translocates to the cell membrane in response to chemoattractant receptor activation and PIP3- and Gβγ stimulation. PKA activity, which inactivates P-Rex1 GEF activity, can also inhibit this translocation. In addition to a negative effect on P-Rex1 GEF activity, the C-terminal domains also serve to keep the protein sequestered in the cytosol. A truncated form of P-Rex1, containing only the tandem DH/PH domains has a higher level of association with the plasma membrane. PIP3 and Gβγ stimulation can increase the membrane association of this P-Rex1 mutant, suggesting that these tandem domains are sufficient for membrane translocation. Membrane-derived P-Rex1 has higher basal activity than cytosolic P-Rex1. .

Role in leukocyte function
P-Rex1 controls neutrophil migration towards sites of inflammation and generation of reactive oxygen species (ROS or superoxide).

P-Rex1 functions to activate Rac2 (and to a lesser extent Rac1) in response to chemoattractant stimulation of GPCRs. Mouse neutrophils that lack P-Rex1 mount an impared NADPH-dependent ROS response to LPS priming and fMLP or C5a stimulation. Similarly, the recruitment of neutrophils to sites of inflammation is severely impaired in mice that lack P-Rex1. Isolated P-Rex1-deficient neutrophils migrate with slightly reduced speed but can orientate themselves in a chemotactic gradient.

In macrophages, P-Rex1 regulates Rac1 activation and chemotaxis. Macrophages that lack P-Rex1 were deficient for ROS production after C5a stimulation and showed decreased Rac1 activation in experiments similar to those described above for neutrophils.

Role in neurons
P-Rex1 is expressed in a range of cells associated with the nervous system and it's expression changes throughout the stages of mouse brain development. In neuronal cell culture studies P-Rex1 has been shown to regulate al migration and neurite differentiation. Furthermore, transfection experiments with dominant-negative P-Rex1-constructs show that P-Rex1 can regulate neuronal migration in the developing mouse nervous system.

Interactions
Gβγ, PIP3, mTOR, PTEN.