The G protein gamma subunit. Requirements for dimerization with beta subunits

Area of Science:

• Molecular Biology
• Protein Biochemistry
• Cell Signaling

Background:

• Guanine nucleotide-binding protein (G protein) beta and gamma subunits form a stable complex essential for G protein function.
• The assembly process of beta gamma subunits is complex and not fully understood, with variations observed depending on the synthesis system.
• Dimerization-competent gamma 2 can be synthesized without specific chaperonins, indicating intrinsic properties facilitate assembly.

Purpose of the Study:

• To identify the specific regions of the gamma 2 subunit necessary for native beta gamma dimer formation.
• To investigate the role of N- and C-terminal sequences in gamma 2 subunit dimerization.
• To assess the impact of N-terminal modifications on beta gamma dimer and heterotrimer assembly.

Main Methods:

• In vitro synthesis of beta 1 and gamma 2 subunits using rabbit reticulocyte lysate and wheat germ extract.
• Generation and synthesis of N- and C-terminal truncations of the gamma 2 subunit.
• Assessment of beta gamma dimerization by measuring the stabilization of beta 1 to tryptic proteolysis.
• Coimmunoprecipitation assays in intact COS cells to study heterotrimer formation with epitope-tagged gamma 2.

Main Results:

• The gamma 2 subunit synthesized in rabbit reticulocyte lysate readily forms beta gamma dimers, unlike when synthesized in wheat germ extract.
• A region between residues 45 and 59 of the gamma 2 subunit is critical for dimerization, as its removal prevents dimer formation.
• N-terminal truncations of 15 amino acids diminished, while 25 amino acids abolished, beta gamma dimer formation, indicating an N-terminal requirement.
• An N-terminal epitope tag did not impede beta gamma dimerization or heterotrimer formation in COS cells.

Conclusions:

• Specific regions at both the N-terminus and internally (residues 45-59) of the gamma 2 subunit are essential for native beta gamma dimer formation.
• The gamma 2 subunit possesses intrinsic properties that facilitate dimerization, independent of external cofactors under certain conditions.
• N-terminal modifications, such as epitope tagging, can be tolerated without disrupting G protein complex assembly.