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Insulin-Like Growth Factor I Controls a Mutually Exclusive Association of RACK1 with Protein Phosphatase 2A and ß1 Integrin To Promote Cell Migration

Cell Biology Laboratory, Department of Biochemistry, BioSciences Institute, National University of Ireland, Cork, Ireland,¹ Gallo Research Center, Department of Neurology, University of California, San Francisco, Emeryville, California 94608²

Received 23 September 2005/ Returned for modification 10 November 2005/ Accepted 17 March 2006

The WD repeat scaffolding protein RACK1 can mediate integration of the insulin-like growth factor I receptor (IGF-IR) and integrin signaling in transformed cells. To address the mechanism of RACK1 function, we searched for regulatory proteins that associate with RACK1 in an IGF-I-dependent manner. The serine threonine phosphatase protein phosphatase 2A (PP2A) was found associated with RACK1 in serum-starved cells, and it dissociated immediately upon stimulation with IGF-I. This dissociation of PP2A from RACK1 and an IGF-I-mediated decrease in cellular PP2A activity did not occur in cells expressing either the serine 1248 or tyrosine 1250/1251 mutants of the IGF-IR that do not interact with RACK1. Recombinant RACK1 could bind to PP2A in vitro and restore phosphatase activity to PP2A from IGF-I-stimulated cells. Ligation of integrins with fibronectin or Matrigel was sufficient to facilitate IGF-I-mediated dissociation of PP2A from RACK1 and also to recruit ß1 integrin as PP2A dissociated. By using TAT-fused N-terminal and C-terminal deletion mutants of RACK1, we determined that both PP2A and ß1 integrin interact in the C terminus of RACK1 within WD repeats 4 to 7. This suggests that integrin ligation displaces PP2A from RACK1. MCF-7 cells overexpressing RACK1 exhibited enhanced motility, which could be reversed by the PP2A inhibitor okadaic acid. Small interfering RNA-mediated suppression of RACK1 also decreased the migratory capacity of DU145 cells. Taken together, our findings indicate that RACK1 enhances IGF-I-mediated cell migration through its ability to exclusively associate with either ß1 integrin or PP2A in a complex at the IGF-IR.

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