Substrate Specificity and Recognition Is Conferred by the Pleckstrin Homology Domain of the Dbl Family Guanine Nucleotide Exchange Factor P-Rex2

An Autoinhibitory Role for the Pleckstrin Homology Domain of Interleukin-2-Inducible Tyrosine Kinase and Its Interplay with Canonical Phospholipid Recognition

Pleckstrin homology (PH) domains are well-known as phospholipid binding modules, yet evidence that PH domain function extends beyond lipid recognition is mounting. In this work, we characterize a protein binding function for the PH domain of interleukin-2-inducible tyrosine kinase (ITK), an immune cell specific signaling protein that belongs to the TEC family of nonreceptor tyrosine kinases. Its N-terminal PH domain is a well-characterized lipid binding module that localizes ITK to the membrane via phosphatidylinositol 3,4,5-trisphosphate (PIP₃) binding. Using a combination of nuclear magnetic resonance spectroscopy and mutagenesis, we have mapped an autoregulatory protein interaction site on the ITK PH domain that makes direct contact with the catalytic kinase domain of ITK, inhibiting the phospho-transfer reaction. Moreover, we have elucidated an important interplay between lipid binding by the ITK PH domain and the stability of the autoinhibitory complex formed by full length ITK. The ITK activation loop in the kinase domain becomes accessible to phosphorylation to the exogenous kinase LCK upon binding of the ITK PH domain to PIP₃. By clarifying the allosteric role of the ITK PH domain in controlling ITK function, we have expanded the functional repertoire of the PH domain generally and opened the door to alternative strategies to target this specific kinase in the context of immune cell signaling.

Regulation and function of P-Rex family Rac-GEFs

The P-Rex family are Dbl-type guanine-nucleotide exchange factors for Rac family small G proteins. They are distinguished from other Rac-GEFs through their synergistic mode of activation by the lipid second messenger phosphatidyl inositol (3,4,5) trisphosphate and the Gβγ subunits of heterotrimeric G proteins, thus acting as coincidence detectors for phosphoinositide 3-kinase and G protein coupled receptor signaling. Work in genetically-modified mice has shown that P-Rex1 has physiological importance in the inflammatory response and the migration of melanoblasts during development, whereas P-Rex2 controls the dendrite morphology of cerebellar Purkinje neurons as well as glucose homeostasis in liver and adipose tissue. Deregulation of P-Rex1 and P-Rex2 expression occurs in many types of cancer, and P-Rex2 is frequently mutated in melanoma. Both GEFs promote tumor growth or metastasis. This review critically evaluates the P-Rex literature and tools available and highlights exciting recent developments and open questions.

Phosphoinositide phosphatases: just as important as the kinases

Phosphoinositide phosphatases comprise several large enzyme families with over 35 mammalian enzymes identified to date that degrade many phosphoinositide signals. Growth factor or insulin stimulation activates the phosphoinositide 3-kinase that phosphorylates phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)] to form phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)], which is rapidly dephosphorylated either by PTEN (phosphatase and tensin homologue deleted on chromosome 10) to PtdIns(4,5)P(2), or by the 5-phosphatases (inositol polyphosphate 5-phosphatases), generating PtdIns(3,4)P(2). 5-phosphatases also hydrolyze PtdIns(4,5)P(2) forming PtdIns(4)P. Ten mammalian 5-phosphatases have been identified, which regulate hematopoietic cell proliferation, synaptic vesicle recycling, insulin signaling, and embryonic development. Two 5-phosphatase genes, OCRL and INPP5E are mutated in Lowe and Joubert syndrome respectively. SHIP [SH2 (Src homology 2)-domain inositol phosphatase] 2, and SKIP (skeletal muscle- and kidney-enriched inositol phosphatase) negatively regulate insulin signaling and glucose homeostasis. SHIP2 polymorphisms are associated with a predisposition to insulin resistance. SHIP1 controls hematopoietic cell proliferation and is mutated in some leukemias. The inositol polyphosphate 4-phosphatases, INPP4A and INPP4B degrade PtdIns(3,4)P(2) to PtdIns(3)P and regulate neuroexcitatory cell death, or act as a tumor suppressor in breast cancer respectively. The Sac phosphatases degrade multiple phosphoinositides, such as PtdIns(3)P, PtdIns(4)P, PtdIns(5)P and PtdIns(3,5)P(2) to form PtdIns. Mutation in the Sac phosphatase gene, FIG4, leads to a degenerative neuropathy. Therefore the phosphatases, like the lipid kinases, play major roles in regulating cellular functions and their mutation or altered expression leads to many human diseases.

P-Rex1 participates in Neuregulin-ErbB signal transduction and its expression correlates with patient outcome in breast cancer

The Neuregulins and their receptors, the ErbB/HER subfamily of receptor tyrosine kinases, have critical roles in animal physiology, and their deregulation is frequent in cancer. Here we report the identification of the guanine nucleotide exchange factor, phosphatidylinositol 3,4,5-triphosphate-dependent Rac exchanger 1 (P-Rex1), as a novel mediator in signalling by ErbB/HER receptors. P-Rex1 was formerly described as a phosphoinositide 3-kinase and Gβγ activated protein that regulates Rac function. We define how ErbB/HER receptors regulate P-Rex1 function, which involves dephosphorylation of inhibitory residues, and phosphorylation of activating residues of P-Rex. The net balance resulting from activation of this phosphorylation/dephosphorylation cycle of P-Rex1 favours Rac activation. Molecular and biological studies indicated that P-Rex1 phosphorylation regulated the proliferation of breast cancer cells, and P-Rex1 knockdown affected their migration or invasiveness, as well as their in vivo tumourigenic potential. Moreover, as we found correlation between high P-Rex1 expression and poor patient outcome in breast cancer, P-Rex1 targeting may be therapeutically relevant in cancer.

Bacterial guanine nucleotide exchange factors SopE-like and WxxxE effectors

Subversion of Rho family small GTPases, which control actin dynamics, is a common infection strategy used by bacterial pathogens. In particular, Salmonella enterica serovar Typhimurium, Shigella flexneri, enteropathogenic Escherichia coli (EPEC), and enterohemorrhagic Escherichia coli (EHEC) translocate type III secretion system (T3SS) effector proteins to modulate the Rho GTPases RhoA, Cdc42, and Rac1, which trigger formation of stress fibers, filopodia, and lamellipodia/ruffles, respectively. The Salmonella effector SopE is a guanine nucleotide exchange factor (GEF) that activates Rac1 and Cdc42, which induce "the trigger mechanism of cell entry." Based on a conserved Trp-xxx-Glu motif, the T3SS effector proteins IpgB1 and IpgB2 of Shigella, SifA and SifB of Salmonella, and Map of EPEC and EHEC were grouped together into a WxxxE family; recent studies identified the T3SS EPEC and EHEC effectors EspM and EspT as new family members. Recent structural and functional studies have shown that representatives of the WxxxE effectors share with SopE a 3-D fold and GEF activity. In this minireview, we summarize contemporary findings related to the SopE and WxxxE GEFs in the context of their role in subverting general host cell signaling pathways and infection.

P-Rex1 links mammalian target of rapamycin signaling to Rac activation and cell migration

Polarized cell migration results from the transduction of extra-cellular cues promoting the activation of Rho GTPases with the intervention of multidomain proteins, including guanine exchange factors. P-Rex1 and P-Rex2 are Rac GEFs connecting Gbetagamma and phosphatidylinositol 3-kinase signaling to Rac activation. Their complex architecture suggests their regulation by protein-protein interactions. Novel mechanisms of activation of Rho GTPases are associated with mammalian target of rapamycin (mTOR), a serine/threonine kinase known as a central regulator of cell growth and proliferation. Recently, two independent multiprotein complexes containing mTOR have been described. mTORC1 links to the classical rapamycin-sensitive pathways relevant for protein synthesis; mTORC2 links to the activation of Rho GTPases and cytoskeletal events via undefined mechanisms. Here we demonstrate that P-Rex1 and P-Rex2 establish, through their tandem DEP domains, interactions with mTOR, suggesting their potential as effectors in the signaling of mTOR to Rac activation and cell migration. This possibility was consistent with the effect of dominant-negative constructs and short hairpin RNA-mediated knockdown of P-Rex1, which decreased mTOR-dependent leucine-induced activation of Rac and cell migration. Rapamycin, a widely used inhibitor of mTOR signaling, did not inhibit Rac activity and cell migration induced by leucine, indicating that P-Rex1, which we found associated to both mTOR complexes, is only active when in the mTORC2 complex. mTORC2 has been described as the catalytic complex that phosphorylates AKT/PKB at Ser-473 and elicits activation of Rho GTPases and cytoskeletal reorganization. Thus, P-Rex1 links mTOR signaling to Rac activation and cell migration.