Simona Paris

p95-APP1 links membrane transport to Rac-mediated reorganization of actin

Motility requires protrusive activity at the cellular edge, where Rho family members regulate actin dynamics. Here we show that p95-APP1 (ArfGAP-putative, Pix-interacting, paxillin-interacting protein 1), a member of the GIT1/PKL family, is part of a complex that interacts with Rac. Wild-type and truncated p95-APP1 induce actin-rich protrusions mediated by Rac and ADP-ribosylation factor 6 (Arf6). Distinct p95-APP1-derived polypeptides have different distributions, indicating that p95-APP1 cycles between the cell surface and endosomes. Our results show that p95-APP1 functionally interacts with Rac and localizes to endosomal compartments, thus identifying p95-APP1 as a molecular link between actin organization, adhesion, and membrane transport during cell motility.

Generation and Characterization of Rac3 Knockout Mice

ABSTRACT Rac proteins are members of the Rho family of GTPases involved in the regulation of actin dynamics. The three highly homologous Rac proteins in mammals are the ubiquitous Rac1, the hematopoiesis-specific Rac2, and the least-characterized Rac3. We show here that Rac3 mRNA is widely and specifically expressed in the developing nervous system, with highest concentration at embryonic day 13 in the dorsal root ganglia and ventral spinal cord. At postnatal day 7 Rac3 appears particularly abundant in populations of projection neurons in several regions of the brain, including the fifth layer of the cortex and the CA1-CA3 region of the hippocampus. We generated mice deleted for the Rac3 gene with the aim of analyzing the function of this GTPase in vivo. Rac3 knockout animals survive embryogenesis and show no obvious developmental defects. Interestingly, specific behavioral differences were detected in the Rac3-deficient animals, since motor coordination and motor learning on the rotarod was superior to that of their wild-type littermates. No obvious histological or immunohistological differences were observed at major sites of Rac3 expression. Our results indicate that, in vivo, Rac3 activity is not strictly required for normal development in utero but may be relevant to later events in the development of a functional nervous system.

Leucine-zipper-mediated homo- and hetero-dimerization of GIT family p95-ARF GTPase-activating protein, PIX-, paxillin-interacting proteins 1 and 2.

ADP-ribosylation factor GTPase-activating proteins (ARFGAPs) of the G-protein-coupled receptor kinase interactor 1/p95 paxillin kinase linker/p95-ARFGAP Pak-interacting exchange factor paxillin-binding protein (APP)-1 family are multidomain proteins, which interact functionally with both ARF and Rac GTPases. These proteins are involved in the dynamic reorganization of adhesion and the cytoskeleton during cell motility. Our previous work [Di Cesare, Paris, Albertinazzi, Dariozzi, Andersen, Mann, Longhi and de Curtis (2000) Nat. Cell Biol. 2, 521-530] has pointed out a role for p95-APP1 in the regulation of ARF6-mediated membrane recycling. These proteins include different domains, and are capable of interacting stably with proteins that are supposed to play a role in the regulation of actin dynamics and adhesion. They contain a coiled-coil region comprising a putative leucine zipper, predicted to be involved in dimerization. In the present study, we have investigated the possibility that these proteins form dimers. Our results show that p95-APP1 forms homodimers and may also form heterodimers with the other member of the family, p95 paxillin kinase linker/p95-APP2. Both homo- and heterodimerization are disrupted by mutation of two leucine residues in the coiled-coil region of p95-APP1. The N-terminal portion of p95-APP1, including the ARFGAP domain, three ankyrin repeats and the Pak-interacting exchange factor-binding region, are not required for dimerization. Evidence is presented for the existence of endogenous oligomeric complexes. The implication of dimerization/oligomerization in the functioning of these proteins is discussed.

Liprin-α1 promotes cell spreading on the extracellular matrix by affecting the distribution of activated integrins

Integrin activation is needed to link the extracellular matrix with the actin cytoskeleton during cell motility. Protrusion requires coordination of actin dynamics with focal-adhesion turnover. We report that the adaptor protein liprin-α1 is stably associated with the cell membrane. Lipin-α1 shows a localization that is distinct from that of activated β1 integrins at the edge of spreading cells. Depletion of liprin-α1 inhibits the spreading of COS7 cells on fibronectin by affecting lamellipodia formation, whereas its overexpression enhances spreading, and lamellipodia and focal-adhesion formation at the cell edge. Cooperation between liprin-α1 and talin is needed, because either talin or liprin depletion prevents spreading in the presence of the other protein. The effects of liprin on spreading, but not its effects in the reorganization of the cell edge, are dependent on its interaction with leukocyte common antigen-related tyrosine phosphatase receptors. Therefore, liprin is an essential regulator of cell motility that contributes to the effectiveness of cell-edge protrusion.

βPIX controls cell motility and neurite extension by regulating the distribution of GIT1

Cell motility entails the reorganization of the cytoskeleton and membrane trafficking for effective protrusion. GIT1/p95-APP1 is a member of a family of GTPase-activating proteins for ARF GTPases that affect endocytosis, adhesion and migration. GIT1 associates with paxillin and a complex including the Rac/Cdc42 exchanging factors PIX/Cool and the kinase PAK. In this study, we show that overexpression of βPIX induces the accumulation of endogenous and overexpressed GIT1 at large structures similar to those induced by an ArfGAP-defective mutant of GIT1 (p95-C2). Immunohistochemical analysis and immunoelectron microscopy reveal that these structures include the endogenous transferrin receptor. Time-lapse analysis during motogenic stimuli shows that the formation and perinuclear accumulation of the p95-C2-positive structures is paralleled by inhibition of lamellipodium formation and cell retraction. Both dimerization and a functional SH3 domain of βPIX are required for the accumulation of GIT1 in fibroblasts, which is prevented by the monomeric PIX-PG-ΔLZ. This mutant also prevents the formation of endocytic aggregates and inhibition of neurite outgrowth in retinal neurons expressing p95-C2. Our results indicate that βPIX is an important regulator of the subcellular distribution of GIT1, and suggest that alteration in the level of expression of the complex affects the endocytic compartment and cell motility.

Analysis of the subcellular distribution of avian p95-APP2, an ARF-GAP orthologous to mammalian paxillin kinase linker

We describe here the identification and characterization of avian p95-APP2, a multi-domain protein of a recently identified family of ADP-ribosylation factor (ARF)-GTPase-activating proteins (GAPs) including mammalian G protein-coupled receptor kinases (GRK)-interactor 1 (GIT1), paxillin kinase linker (PKL), and GIT2, as well as avian p95-APP1. The p95-APP2 is eluted from Rac-GTP-gamma-S, but not from Rac-GDP-beta-S columns. As other members of the family, p95-APP2 has binding regions for the focal adhesion protein paxillin, and for the Rac exchanging factor PIX. Sequence comparison indicates that p95-APP2 is the avian orthologue of mammalian PKL. Expression studies showed a largely diffuse distribution of the full length p95-APP2, without evident effects on cell morphology. We observed a dramatic difference between the localization of the amino-terminal portion of the protein, including the ARF-GAP domain and the three ankyrin repeats, and the carboxy-terminal portion including the paxillin-binding site. Moreover, the expression of truncated carboxy-terminal polypeptides including both the PIX- and paxillin-binding regions leads to a marked localization of the protein together with paxillin at large vesicles. Comparison of the expression of corresponding ARF-GAP-deficient constructs from p95-APP2 and p95-APP1 shows their distribution at distinct endocytic compartments. Altogether, these data support a role of distinct members of this family of ARF-GAPs in the regulation of different steps of membrane traffic during cell motility, and suggest that p95-APP2 may shuttle between an intracellular compartment and the cell periphery, although, further work will be needed to address this point.

Biochemical and functional characterization of the interaction between liprin-α1 and GIT1: Implications for the regulation of cell motility

We have previously identified the scaffold protein liprin-α1 as an important regulator of integrin-mediated cell motility and tumor cell invasion. Liprin-α1 may interact with different proteins, and the functional significance of these interactions in the regulation of cell motility is poorly known. Here we have addressed the involvement of the liprin-α1 partner GIT1 in liprin-α1-mediated effects on cell spreading and migration. GIT1 depletion inhibited spreading by affecting the lamellipodia, and prevented liprin-α1-enhanced spreading. Conversely inhibition of the formation of the liprin-α1-GIT complex by expression of liprin-ΔCC3 could still enhance spreading, although to a lesser extent compared to full length liprin-α1. No cumulative effects were observed after depletion of both liprin-α1 and GIT1, suggesting that the two proteins belong to the same signaling network in the regulation of cell spreading. Our data suggest that liprin-α1 may compete with paxillin for binding to GIT1, while binding of βPIX to GIT1 was unaffected by the presence of liprin-α1. Interestingly, GIT and liprin-α1 reciprocally regulated their subcellular localization, since liprin-α1 overexpression, but not the GIT binding-defective liprin-ΔCC3 mutant, affected the localization of endogenous GIT at peripheral and mature central focal adhesions, while the expression of a truncated, active form of GIT1 enhanced the localization of endogenous liprin-α1 at the edge of spreading cells. Moreover, GIT1 was required for liprin-α1-enhanced haptotatic migration, although the direct interaction between liprin-α1 and GIT1 was not needed. Our findings show that the functional interaction between liprin-α1 and GIT1 cooperate in the regulation of integrin-dependent cell spreading and motility on extracellular matrix. These findings and the possible competition of liprin-α1 with paxillin for binding to GIT1 suggest that alternative binding of GIT1 to either liprin-α1 or paxillin plays distinct roles in different phases of the protrusive activity in the cell.

Assay and properties of the GIT1/p95-APP1 ARFGAP

GIT1/p95-APP1 is an adaptor protein with an aminoterminal ARFGAP domain involved in the regulation of ARF6 function. GIT1/p95-APP1 forms stable complexes with a number of proteins including downstream effectors and exchanging factors for members of the Rho family of small GTPases. This protein can also interact with other adaptor proteins implicated in the regulation of cell adhesion and synapse formation. The stability of the endogenous and reconstituted complexes after cell lysis allows the biochemical identification and characterization of the GIT1 complexes that can be isolated from different cell types. This article presents methods for the identification of the endogenous and reconstituted GIT1 complexes that can be utilized for the biochemical and functional characterization of the complexes from different tissue and cell types.