A subunit interaction site in human luteinizing hormone: identification by photoaffinity cross-linking

Area of Science:

• Biochemistry
• Molecular Biology
• Endocrinology

Background:

• Glycoprotein hormones (LH, hCG, FSH, TSH) consist of alpha and beta subunits.
• Understanding subunit association is crucial for hormone structure and stability.
• Previous methods for identifying interaction sites have limitations.

Purpose of the Study:

• To define the noncovalent association sites between alpha and beta subunits of glycoprotein hormones.
• To map the three-dimensional structure and explain heterodimer stability.
• To identify complementary contact sites on respective subunits using photoaffinity cross-linking.

Main Methods:

• Incorporation of a photoaffinity ligand (Bpa) into a specific beta-subunit peptide (hLH beta sequence 1-15).
• Photo-cross-linking of the labeled beta peptide with the alpha subunit under UV light.
• Isolation and analysis of the cross-linked alpha-fraction using peptide mapping and sequence analysis.

Main Results:

• A Bpa-labeled beta (1-15) peptide specifically cross-linked to the alpha subunit.
• The contact site on the alpha subunit was localized to the N-terminal fragment (18-33), likely Met-29 or Gly-30.
• Other beta-fragments (receptor-binding loop and CAGY sequence peptide) did not cross-link.

Conclusions:

• The alpha and beta subunit contact sites are adjacent to or overlap with receptor-binding regions.
• This proximity suggests a multicomponent receptor-binding domain essential for hormone activity.
• Defines key interaction points for glycoprotein hormone structure-function relationships.