Dynamic aspects of the incorporation of proteins into biological membranes

The contributions of intramembranous and extramembranous segments of transmembrane proteins to frictional forces have been studied by covalently attached 14N- and 15N-indane dione and maleimide spin labels using saturation transfer electron spin resonance spectroscopy. The role of molecular size and membrane viscosity is discussed in determining rotational mobilities of proteins. By comparing the measured rotational correlation times with the predictions of hydrodynamic models the aggregation states of transmembrane proteins is estimated. On increasing the viscosity of the aqueous phase by polyols the viscous drag of the extramembranous segments of proteins is increased and from systematic hydrodynamic measurements the size of the protruding segments can be estimated. The role of slowed molecular diffusion is briefly discussed in the inhibition of enzymatic activity.