Editors' ChoiceCell Biology

Blocking Integrin Activation at the α Subunit

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Science Signaling  08 Nov 2011:
Vol. 4, Issue 198, pp. ec309
DOI: 10.1126/scisignal.4198ec309

Integrins are heterodimers of α and β subunits that control cellular adhesion to extracellular matrix proteins. The activity state of integrins is dynamically regulated to enable cells to migrate, divide, or remain stationary depending on the context and extracellular signals. Integrins can adopt three conformations—two that are inactive with low to moderate ligand affinity and one with high affinity for ligand that is considered the active state—and the active and inactive conformations can be detected with specific antibodies (see Bass). The currently known regulators of integrin conformation interact with the cytosolic domain of the β subunit to either promote the open state or inhibit the binding of proteins that promote this state. Rantala et al. identified SHARPIN as an inhibitor of integrin activation that bound to the α subunit. Knockdown of SHARPIN increased the binding of antibodies specific for active β1 integrins, while reducing binding of antibodies specific for inactive β1 integrins. Overexpression of SHARPIN decreased active β1 integrins. SHARPIN knockdown also increased the rate of cell migration on low concentrations of matrix and increased cell adhesion to fibronectin. In vitro pull-down assays showed that SHARPIN bound to peptides from the α1 or α2 cytoplasmic tail but not those from the β1 tail, and this interaction was also detected in cultured cells by fluorescence resonance energy transfer (FRET) between labeled proteins. SHARPIN colocalized with inactive β1 integrins in membrane ruffles but did not colocalize with active β1 at sites of matrix adhesion. Coimmunoprecipitation of SHARPIN with α integrins was increased in cells in suspension compared with that in cells attached to dishes, consistent with enhanced binding to unoccupied integrins. FRET and proximity ligation assays indicated that SHARPIN interfered with the binding of the activating proteins kindlin and talin to the β1 subunit. Analysis of mice or cells from mice with a mutation in the SHARPIN-encoding gene showed that a subpopulation of keratinocytes exhibited enhanced binding to the antibody that recognizes active β1 integrins, and isolated splenocytes exhibited increased active β1 integrins and binding of an integrin ligand. Thus, SHARPIN appears to be a key regulator of integrin activity and does so through an interaction with the α subunit.

J. K. Rantala, J. Pouwels, T. Pellinen, S. Veltel, P. Laasola, E. Mattila, C. S. Potter, T. Duffy, J. P. Sundberg, O. Kallioniemi, J. A. Askari, M. J. Humphries, M. Parsons, M. Salmi, J. Ivaska, SHARPIN is an endogenous inhibitor of β1-integrin activation. Nat. Cell Biol. 13, 1315–1324 (2011). [PubMed]

M. D. Bass, SHARPINing integrin inhibition. Nat. Cell Biol. 13, 1292–1293 (2011). [PubMed]

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