Practice question

Diffusion behavior of membrane proteins governed by hydrodynamic and cytoskeletal factors. Saffman-Delbruck model relates diffusion coefficient D to membrane thickness h viscosity mu m and protein radius r through D proportional to natural log of membrane to protein size. Number of transmembrane domains increases effective radius and frictional drag because each helix engages annular lipids diffusing as complex and more extensive hydrophobic surface contacts viscous acyl chains. Single-pass glycophorin diffusion coefficient about five times ten to minus nine centimeters squared per second faster than seven-pass GPCR about one times ten to minus nine and fourteen-pass Band 3 about zero point five. Additional slowing originates from hop diffusion model developed by Kusumi showing actin cytoskeleton fences create compartments eighty to two hundred nanometers where proteins diffuse rapidly then hop between compartments limited by transient fence and picket interactions. Binding to immobilized ankyrin spectrin network reduces mobile fraction to fifty percent. Size of extracellular loops influences extracellular matrix interactions but less than transmembrane count, hydropathy index predicts insertion, pH charge limited effect. Number of transmembrane domains therefore primarily affects mobility.