Subunit structure and dynamics of the insulin receptor

Area of Science:

• Biochemistry
• Cell Biology
• Molecular Endocrinology

Background:

• The insulin receptor mediates cellular responses to insulin, a key metabolic hormone.
• Understanding the insulin receptor's structure is crucial for elucidating insulin signaling pathways.
• Previous studies suggested a heterotetrameric structure, but detailed subunit interactions and stoichiometry remained unclear.

Purpose of the Study:

• To elucidate the minimum subunit composition and stoichiometry of the physiologically relevant insulin receptor.
• To propose a structural model for the insulin receptor based on experimental evidence.
• To investigate the structural similarities and differences between insulin and insulin-like growth factor I (IGF-I) receptors.

Main Methods:

• Affinity labeling of the insulin receptor in various cell types and species.
• Analysis of receptor subunit composition and molecular weight using SDS-PAGE.
• Investigation of disulfide bond sensitivity to reduction.
• Proteolytic cleavage studies of receptor subunits.
• Comparative affinity labeling of insulin and IGF-I receptors.

Main Results:

• The insulin receptor is a symmetrical disulfide-linked heterotetramer of two alpha (125 kDa) and two beta (90 kDa) glycoprotein subunits: (beta-S-S-alpha)-S-S-(alpha-S-S-beta).
• Two classes of disulfide bonds were identified: Class I (inter-half) are more sensitive to reduction than Class II (intra-half).
• Reduction of Class I disulfides preserves insulin binding and biological activity.
• The beta subunit is cleaved by proteases, yielding a 45 kDa fragment (beta 1) and a free fragment (beta 2).
• Insulin binding induces conformational changes in the receptor, altering alpha subunit susceptibility to trypsin.
• A distinct receptor with high IGF-I affinity and low insulin affinity shares structural features with the insulin receptor.

Conclusions:

• The proposed model of the insulin receptor as a symmetrical heterotetramer with specific disulfide linkages provides a framework for understanding its function.
• The differential sensitivity of disulfide bonds suggests distinct roles in receptor assembly and signaling.
• Proteolytic cleavage of the beta subunit may be involved in signal transduction.
• Shared structural features between insulin and IGF-I receptors suggest conserved transmembrane signaling mechanisms.