The p85 regulatory subunit of phosphoinositide 3-kinase down-regulates IRS-1 signaling via the formation of a sequestration complex.
Phosphoinositide (PI) 3-kinase is required for most insulin and insulin-like growth factor (IGF) 1-dependent cellular responses. The p85 regulatory subunit of PI 3-kinase is required to mediate the insulin-dependent recruitment of PI 3-kinase to the plasma membrane, yet mice with reduced p85 expression have increased insulin sensitivity. To further understand the role of p85, we examined IGF-1-dependent translocation of p85alpha by using a green fluorescence protein (GFP)-tagged p85alpha (EGFP-p85alpha). In response to IGF-1, but not to PDGF signaling, EGFP-p85alpha translocates to discrete foci in the cell. These foci contain the insulin receptor substrate (IRS) 1 adaptor molecule, and their formation requires the binding of p85 to IRS-1. Surprisingly, monomeric p85 is preferentially localized to these foci compared with the p85-p110 dimer, and these foci are not sites of phosphatidylinositol-3,4,5-trisphosphate production. Ultrastructural analysis reveals that p85-IRS-1 foci are cytosolic protein complexes devoid of membrane. These results suggest a mechanism of signal down-regulation of IRS-1 that is mediated by monomeric p85 through the formation of a sequestration complex between p85 and IRS-1.
Luo
Ji
J
Department of Systems Biology, Harvard Medical School, Beth Israel Deaconess Medical Center, Boston, MA 02115, USA.
Field
Seth J
SJ
Lee
Jennifer Y
JY
Engelman
Jeffrey A
JA
Cantley
Lewis C
LC
D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D003600:Cytosol / Q000378:metabolism; D019281:Dimerization; D015536:Down-Regulation; D049452:Green Fluorescent Proteins / Q000235:genetics; D006801:Humans; D055504:Insulin Receptor Substrate Proteins; D051379:Mice; D019869:Phosphatidylinositol 3-Kinases / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D010750:Phosphoproteins / Q000378:metabolism / Q000502:physiology*; D010766:Phosphorylation; D011485:Protein Binding; D021122:Protein Subunits / Q000378:metabolism; D021381:Protein Transport; D017526:Receptor, IGF Type 1 / Q000819:agonists; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D015398:Signal Transduction; D014443:Tyrosine / Q000378:metabolism
C526292:IRS1 protein, human; D055504:Insulin Receptor Substrate Proteins; C526295:Irs1 protein, mouse; D010750:Phosphoproteins; D021122:Protein Subunits; D011993:Recombinant Fusion Proteins; C485184:enhanced green fluorescent protein; D049452:Green Fluorescent Proteins; D014443:Tyrosine; D019869:Phosphatidylinositol 3-Kinases; D017526:Receptor, IGF Type 1