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Disrupting integrin transmembrane domain heterodimerization increases ligand binding affinity, not valency or
Bing-Hao Luo1, Christopher V Carman, Junichi Takagi
1The CBR Institute for Biomedical Research and Department of Pathology, Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115, USA.
Summary
Researchers identified key residues in integrin transmembrane domains affecting ligand binding. Mutations enhancing binding primarily increase monomeric affinity, not receptor valency or clustering.
Area of Science:
- Biochemistry
- Molecular Biology
- Cell Biology
Background:
- Integrins are crucial cell surface receptors involved in cell adhesion and signaling.
- The transmembrane (TM) domains of integrin alpha(IIb)- and beta(3)-subunits mediate receptor assembly and regulation.
- Understanding the TM domain interface is key to deciphering integrin activation mechanisms.
Purpose of the Study:
- To identify specific residues within the integrin alpha(IIb)beta(3) transmembrane domains critical for regulating ligand binding.
- To differentiate between affinity-based and valency-based mechanisms of integrin activation.
- To elucidate how TM domain interactions influence the integrin's conformation and signaling.
Main Methods:
- Site-directed mutagenesis was employed, substituting non-leucine residues in TM domains with leucine.
- Functional assays measured ligand binding affinity and valency using monovalent and multivalent ligands.
- Confocal microscopy assessed cell-surface clustering of integrin mutants.
- Specific mutations like alpha(IIb) W967C were used to create tetrameric models for valency studies.
Main Results:
- Mutations at specific sites (e.g., alpha(IIb) G972, G976, T981 and beta(3) I693, G708) significantly increased ligand binding.
- A tetrameric mutant (alpha(IIb) W967C) selectively enhanced binding to multivalent ligands, modeling valency-based activation.
- Activating leucine mutants increased binding to monovalent ligands and LIBS epitope exposure, indicating partial extension.
- The beta(3) G708N mutation enhanced ligand binding affinity without affecting cell-surface clustering.
Conclusions:
- Disrupting the integrin TM helix-helix interface primarily increases monomeric ligand binding affinity.
- Integrin activation by TM domain mutations is mainly mediated by affinity enhancement, not increased receptor valency or clustering.
- These findings provide critical insights into the structural basis of integrin activation and regulation.