Functional interaction between bovine rhodopsin and G protein transducin

Area of Science:

• Molecular Biology
• Biochemistry
• G Protein-Coupled Receptors

Background:

• Rhodopsin initiates visual signal transduction by activating G proteins.
• Specific coupling between rhodopsin and transducin (G(t)) is crucial for vision.
• Understanding these interactions is key to deciphering visual processing mechanisms.

Purpose of the Study:

• To identify the regions in bovine rhodopsin responsible for specific coupling with G(t).
• To investigate the role of cytoplasmic loops and G protein C-termini in coupling specificity.
• To elucidate the molecular basis for selective activation of G(t) over other G proteins.

Main Methods:

• Construction and analysis of bovine rhodopsin mutants with swapped cytoplasmic loops (loop 2 and 3) from scallop rhodopsin.
• Preparation of Galpha(i) mutants with replaced C-terminal amino acids from Galpha(t), Galpha(o), and Galpha(q).
• Assays to measure the activation abilities of these mutants for G(t), G(o), and G(i).

Main Results:

• Replacing loop 2 of bovine rhodopsin with scallop rhodopsin's loop reduced G(t) and G(o) activation by ~40%.
• Replacing loop 3 significantly enhanced G(o) activation (4-fold) while decreasing G(t) activation by 60%.
• Loop 3 of bovine rhodopsin discriminates between G(t) and G(o) based on a specific 6-amino acid sequence (region A) in the G protein alpha-subunit.

Conclusions:

• Loop 3 of bovine rhodopsin is critical for specific coupling and activation of the G protein transducin (G(t)).
• The interaction between bovine rhodopsin's loop 3 and region A of Galpha(t) is essential for G(t) activation.
• This interaction mechanism explains the selective activation of G(t) by bovine rhodopsin.