H-Ras forms dimers on membrane surfaces via a protein-protein interface

Area of Science:

• Cellular Biology
• Biochemistry
• Molecular Biology

Background:

• The small GTPase Ras is a key signaling molecule in mammalian cells.
• Ras organization within cell membranes is thought to regulate its activation.
• Lipid anchors (palmitoyl and farnesyl) are implicated in Ras membrane organization.

Purpose of the Study:

• To investigate the mechanism and properties of H-Ras dimerization on membrane surfaces.
• To determine if lipid anchors or protein-protein interactions drive H-Ras organization.
• To characterize the H-Ras dimer interface and its functional implications.

Main Methods:

• Utilized tethered H-Ras constructs on supported lipid bilayers.
• Employed advanced biophysical techniques: fluorescence correlation spectroscopy, photon counting histogram analysis, time-resolved fluorescence anisotropy, single-molecule tracking, and step photobleaching analysis.
• Introduced a Y64A point mutation in the switch II region to probe the dimer interface.

Main Results:

• H-Ras forms dimers on membrane surfaces through a direct protein-protein binding interface.
• The switch II region, near the nucleotide binding cleft, is critical for dimer formation.
• Dimerization is mediated by protein interactions, not lipid anchor clustering.
• Quantified 2D dimerization dissociation constant (Kd) as approximately 1 × 10^3 molecules/µm².
• H-Ras exists as monomers in solution at comparable densities.

Conclusions:

• H-Ras dimerization is a fundamental property occurring on membrane surfaces.
• The switch II region plays a crucial role in mediating H-Ras dimerization.
• Ras dimerization is driven by protein interactions, independent of lipid anchor aggregation.
• These findings provide insights into Ras signaling regulation at the membrane level.