Functional homodimeric glycoprotein hormones: implications for hormone action and evolution

Area of Science:

• Biochemistry
• Molecular Biology
• Endocrinology

Background:

• Glycoprotein hormones like hCG, lutropin, follitropin, and thyrotropin are alphabeta heterodimers essential for reproduction and thyroid function.
• Their structure involves alpha and beta subunits with distinct loops (e.g., alpha2, beta2) stabilized by specific interactions.
• Understanding hormone-receptor interactions is crucial for developing fertility agents.

Purpose of the Study:

• To investigate the structural requirements for glycoprotein hormone activity by creating homodimeric analogs.
• To identify the key regions involved in receptor binding and signal transduction.
• To explore the evolutionary origins of glycoprotein hormone heterodimerization.

Main Methods:

• Engineered homodimeric analogs by swapping alpha2 and beta2 subunits of glycoprotein hormones.
• Incorporated amino-terminal coiled-coil dimerization domains in some constructs.
• Assessed the biological activity and signal transduction of the engineered homodimers.

Main Results:

• Homodimers containing the beta1, alpha2, beta3 loops, with or without the 'seatbelt' residues, retained significant signal transduction activity, albeit with reduced potency compared to hCG.
• Homodimers with alpha1, beta2, alpha3 loops were inactive.
• The beta1, alpha2, beta3 containing homodimers showed 100-1000 fold higher activity than other minimized analogs.

Conclusions:

• The beta1, alpha2, beta3 region of glycoprotein hormones appears to be the primary determinant for receptor contact and activity.
• The conversion of heterodimers to functional homodimers supports an evolutionary model of gene duplication from an active homodimeric ancestor.
• Protein minimization strategies, like those used here, can be applied to other architecturally related protein families.