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Definition of EGF-like, closely interacting modules that bear activation epitopes in integrin beta subunits
J Takagi1, N Beglova, P Yalamanchili
1Center for Blood Research and Department of Pathology, Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115, USA.
Summary
New analysis redefines integrin beta subunit repeats, revealing homology to EGF-like domains and a novel disulfide bond pattern crucial for signal propagation.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- Integrin beta subunits possess cysteine-rich repeats in their extracellular stalk, vital for signal transmission.
- Antibodies targeting integrin activation epitopes often bind to these repeats.
Purpose of the Study:
- To re-evaluate the boundaries of integrin beta subunit repeats.
- To investigate the sequence homology and disulfide bonding within these repeats.
- To understand the structural basis for signal propagation in integrins.
Main Methods:
- Sequence analysis of integrin-like repeats.
- Prediction of disulfide bond connectivity.
- Expression and refolding of truncated beta2 integrin stalk fragments and peptides.
- Reactivity testing of refolded fragments with monoclonal antibodies (mAbs).
Main Results:
- Redefined repeat boundaries reveal statistically significant homology to epidermal growth factor-like (EGF-like) domains.
- A novel disulfide bond connectivity is predicted for the eight cysteines.
- Truncated beta2 integrin stalk fragments expressed at new boundaries were secreted effectively.
- A refolded module 2+3 fragment showed reactivity with activation epitope mAbs, indicating a native fold.
Conclusions:
- The revised repeat structure and predicted disulfide bonds offer new insights into integrin stalk function.
- The tight intermodule connection, with only one intervening residue, supports a role in signal propagation.
- These findings are critical for understanding how integrins transmit signals from the membrane to the ligand-binding site.