Recruitment of the Adaptor Protein Grb2 to EGFR Tetramers
Author(s) -
Noga Kozer,
Dipak Barua,
Christine Henderson,
Edouard C. Nice,
Antony W. Burgess,
William S. Hlavacek,
Andrew H. A. Clayton
Publication year - 2014
Publication title -
biochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.43
H-Index - 253
eISSN - 1520-4995
pISSN - 0006-2960
DOI - 10.1021/bi500182x
Subject(s) - grb2 , signal transducing adaptor protein , förster resonance energy transfer , epidermal growth factor receptor , chemistry , microbiology and biotechnology , erbb3 , cancer research , biology , receptor , signal transduction , biochemistry , fluorescence , physics , quantum mechanics
Adaptor protein Grb2 binds phosphotyrosines in the epidermal growth factor (EGF) receptor (EGFR) and thereby links receptor activation to intracellular signaling cascades. Here, we investigated how recruitment of Grb2 to EGFR is affected by the spatial organization and quaternary state of activated EGFR. We used the techniques of image correlation spectroscopy (ICS) and lifetime-detected Förster resonance energy transfer (also known as FLIM-based FRET or FLIM-FRET) to measure ligand-induced receptor clustering and Grb2 binding to activated EGFR in BaF/3 cells. BaF/3 cells were stably transfected with fluorescently labeled forms of Grb2 (Grb2-mRFP) and EGFR (EGFR-eGFP). Following stimulation of the cells with EGF, we detected nanometer-scale association of Grb2-mRFP with EGFR-eGFP clusters, which contained, on average, 4 ± 1 copies of EGFR-eGFP per cluster. In contrast, the pool of EGFR-eGFP without Grb2-mRFP had an average cluster size of 1 ± 0.3 EGFR molecules per punctum. In the absence of EGF, there was no association between EGFR-eGFP and Grb2-mRFP. To interpret these data, we extended our recently developed model for EGFR activation, which considers EGFR oligomerization up to tetramers, to include recruitment of Grb2 to phosphorylated EGFR. The extended model, with adjustment of one new parameter (the ratio of the Grb2 and EGFR copy numbers), is consistent with a cluster size distribution where 2% of EGFR monomers, 5% of EGFR dimers, <1% of EGFR trimers, and 94% of EGFR tetramers are associated with Grb2. Together, our experimental and modeling results further implicate tetrameric EGFR as the key signaling unit and call into question the widely held view that dimeric EGFR is the predominant signaling unit.