Protein Diffusion in the Membrane

Lateral protein diffusion is the sideways movement of proteins within the lipid bilayer. At physiological temperatures, the membrane acts as a two-dimensional fluid, allowing proteins, lipids, and other components to move freely. This diffusion occurs because the membrane maintains fluidity, enabling proteins to relocate across the cell surface while remaining embedded in the bilayer structure.

The actin cytoskeleton and associated transmembrane proteins form fence-like structures called corrals that compartmentalize the membrane and obstruct free protein movement between regions. However, actin turnover creates temporary gaps in these fences, allowing proteins to occasionally move from one corral to another. This mechanism regulates protein distribution and prevents unrestricted lateral diffusion across the entire cell surface.

Fluorescence recovery after photobleaching (FRAP) is a technique that labels integral membrane proteins with fluorescent probes, then uses a laser to create a non-fluorescent bleached area. As unbleached proteins diffuse into the bleached region, fluorescence gradually recovers. The recovery rate and extent indicate the diffusion coefficient and mobility of membrane proteins, revealing how quickly and extensively they move laterally.

Mouse and human cells were fused to create hybrid cells with human proteins marked red and mouse proteins marked green. Initially, colors remained on separate hemispheres, but within 40 minutes, red and green fluorescence became completely intermixed across the entire cell surface. This demonstrated that membrane proteins actively diffuse laterally and redistribute throughout the fused cell membrane.

Membrane proteins diffuse significantly slower than lipids due to their larger size and structural complexity. For example, lipids traverse a 20-micrometer eukaryotic cell in approximately 20 seconds, while proteins require 600-3600 seconds. FRAP studies revealed that not all membrane proteins are equally mobile, and diffusion rates vary based on protein location, structure, and their diffusion coefficient.

Each membrane protein has a distinct diffusion coefficient determined by its location and structural properties. The extracellular matrix can interact with a protein's extracellular domain and impede movement, while intracellular factors like cytoskeletal fences also restrict mobility. Advanced techniques such as single-particle tracking and optical tweezers help researchers explore how cytoplasmic and exoplasmic protein domains affect lateral diffusion rates.

Researchers employ single-particle tracking to follow individual proteins, optical tweezers to study membrane barriers, and genetically modified proteins to examine the role of cytoplasmic and exoplasmic domains in lateral diffusion. These methods provide detailed insights into how multi pass transmembrane proteins and other membrane components move and interact within the dynamic lipid environment.