Constraint of the translational diffusion of a membrane glycoprotein by its external domains

The translational diffusion of wild-type and underglycosylated molecules of a membrane-integral glycoprotein the Ld class I major histocompatibility complex (MHC) antigen has been measured. The Ld mutant molecules, which lack one or more glycosylation sites, had larger translational diffusion coefficients, D, than did wild-type Ld molecules glycosylated at three sites. The increase in D is linear with loss of glycosylation. The highest value of D approaches that for translational diffusion of molecules constrained only by viscosity of the membrane lipid bilayer. These results indicate that the external portions of cell surface glycoproteins interact significantly with other nearby molecules.