Light-activated rhodopsin induces structural binding motif in G protein alpha subunit

Area of Science:

• Molecular Biology
• Structural Biology
• Cell Signaling

Background:

• Transmembrane receptors initiate cellular signaling by activating heterotrimeric GTP-binding proteins (G proteins).
• The precise mechanism by which receptors recognize and activate G protein alpha subunits remains largely unknown.
• Understanding this interface is crucial for deciphering signal amplification pathways.

Purpose of the Study:

• To determine the three-dimensional structure of the transducin (Gt) alpha subunit C-terminal undecapeptide (Gtα(340-350)) when bound to photoexcited rhodopsin.
• To elucidate the molecular mechanism of G protein activation by light-activated receptors.

Main Methods:

• Determined the structure of Gtα(340-350) using transferred nuclear Overhauser effect (trNOE) spectroscopy.
• Studied the Gtα peptide in complex with photoexcited rhodopsin.
• Docked the NMR structure to the GDP-bound Gt crystal structure for mechanistic insights.

Main Results:

• Photoactivated rhodopsin induced a significant conformational change in Gtα(340-350), transitioning from a disordered state to a helical turn and reverse turn motif.
• A continuous helix formed over residues 325-346, capped by a unique hydrophobic cluster, upon rhodopsin activation.
• This structural rearrangement at the receptor-G protein interface facilitates nucleotide exchange and signal initiation.

Conclusions:

• Photoexcited rhodopsin promotes specific conformational changes in the Gt alpha subunit C-terminus.
• A detailed molecular mechanism for receptor-mediated G protein activation has been demonstrated.
• This study provides critical structural insights into the initiation of G protein signaling cascades.