title: Scaffold proteins IRSp53 and spinophilin regulate localized Rac activation by T-lymphocyte invasion and metastasis protein 1 (TIAM1).
abstract: The Rac exchange factor Tiam1 is involved in diverse cell functions and signaling pathways through multiple protein interactions, raising the question of how signaling and functional specificity are achieved. We have shown that Tiam1 interactions with different scaffold proteins activate different Rac-dependent pathways by recruiting specific Rac effector proteins, and reasoned that there must be regulatory mechanisms governing each interaction. Fibroblasts express at least two Tiam1-interacting proteins, insulin receptor substrate protein 53 kDa (IRSp53) and spinophilin. We used fluorescent resonance energy transfer (FRET) to measure localized Rac activation associated with IRSp53 and spinophilin complexes in individual fibroblasts to test this hypothesis. Pervanadate or platelet-derived growth factor induced localized Rac activation dependent on Tiam1 and IRSp53. Forskolin or epinephrine induced localized Rac activation dependent on Tiam1 and spinophilin. In spinophilin-deficient cells, Tiam1 co-localized with IRSp53 in response to pervanadate or platelet-derived growth factor. In IRSp53-deficient cells, Tiam1 co-localized with spinophilin in response to forskolin or epinephrine. Total cellular levels of activated Rac were affected only in cells with exogenous Tiam1, and were primarily increased in the membrane fraction. Downstream effects of Rac activation were also stimulus and scaffold-specific. Cell ruffling, spreading, and cell adhesion were dependent on IRSp53, but not spinophilin. Epinephrine decreased IRSp53-dependent adhesion and increased cell migration in a Rac and spinophilin-dependent fashion. These results support the idea that Tiam1 interactions with different scaffold proteins couple distinct upstream signals to localized Rac activation and specific downstream pathways, and suggest that manipulating Tiam1-scaffold interactions can modulate Rac-dependent cellular behaviors.
authors.affiliation: Molecular Oncology Research Institute, Tufts Medical Center, Tufts UniversitySackler School of Graduate Biomedical Sciences, Boston, Massachusetts 02111, USA.
authors.forename: Soumitra
authors.initials: S
authors.lastname: Rajagopal
authors.forename: Yuxin
authors.initials: Y
authors.lastname: Ji
authors.forename: Kun
authors.initials: K
authors.lastname: Xu
authors.forename: Yuhuan
authors.initials: Y
authors.lastname: Li
authors.forename: Kathleen
authors.initials: K
authors.lastname: Wicks
authors.forename: Jiewei
authors.initials: J
authors.lastname: Liu
authors.forename: Ka-Wing
authors.initials: KW
authors.lastname: Wong
authors.forename: Ira M
authors.initials: IM
authors.lastname: Herman
authors.forename: Ralph R
authors.initials: RR
authors.lastname: Isberg
authors.forename: Rachel J
authors.initials: RJ
authors.lastname: Buchsbaum
mesh_terms: D000199:Actins / Q000737:chemistry; D000818:Animals; D002448:Cell Adhesion; D002465:Cell Movement; D004837:Epinephrine / Q000494:pharmacology; D005347:Fibroblasts / Q000378:metabolism; D031541:Fluorescence Resonance Energy Transfer / Q000379:methods; D005786:Gene Expression Regulation; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D006801:Humans; D051379:Mice; D008840:Microfilament Proteins / Q000737:chemistry*; D041681:NIH 3T3 Cells; D009419:Nerve Tissue Proteins / Q000737:chemistry*; D000075684:T-Lymphoma Invasion and Metastasis-inducing Protein 1; D020744:rac GTP-Binding Proteins / Q000378:metabolism*; D000099273:Neurabins
chemical_list: D000199:Actins; D004837:Epinephrine; D020662:Guanine Nucleotide Exchange Factors; D008840:Microfilament Proteins; D009419:Nerve Tissue Proteins; D000075684:T-Lymphoma Invasion and Metastasis-inducing Protein 1; D020744:rac GTP-Binding Proteins; D000099273:Neurabins; C119539:BAIAP2 protein, human; C539986:Baiap2 protein, mouse; C087334:TIAM1 protein, human; C513260:Tiam1 protein, mouse