Interface analysis of small GTP binding protein complexes suggests preferred membrane orientations

Area of Science:

• Structural biology
• Biochemistry
• Molecular biology

Background:

• Small GTP binding proteins (GTPases) are key regulators of cellular processes.
• Their function relies on interactions with effectors and regulators, primarily through switch I and II regions.
• A specific 'flat' side of the G domain, including helix α4 and α5, is notably devoid of contacts.

Purpose of the Study:

• To investigate the structural basis and potential function of the uncontacted 'flat' side of the GTPase G domain.
• To understand how this region might interact with membranes or participate in regulatory mechanisms.

Main Methods:

• Analysis of crystal structures of small GTP binding protein complexes.
• Interface analysis of GTPase complexes in different nucleotide-bound states (GTP and GDP).

Main Results:

• The 'flat' side (helix α4 and α5) is consistently avoided by interacting proteins, except for Ran.
• This region is proposed to facilitate simultaneous membrane interaction (via lipidated C-terminus and α4 helix charges) and effector/regulator binding.
• Ras dimerization occurs at this 'flat' side, suggesting a regulatory role.
• GTPase complexes exhibit different flexibilities depending on the bound nucleotide.

Conclusions:

• The 'flat' side of the GTPase G domain represents a potentially important site for membrane association and regulation.
• Understanding these structural features provides insights into GTPase function and regulation, particularly in membrane-associated signaling pathways.