Cell membranes often are patchy, composed of lateral domains. These domains may be formed by barriers within or on either side of the membrane bilayer. Major histocompatibility complex (MHC) class 1 molecules that were either transmembrane- (H-2Db) or glycosylphosphatidylinositol (GPI)-anchored (Qa2) were labeled with antibody-coated gold particles and moved across the cell surface with a laser optical tweezers until they encountered a barrier, the barrier-free path length (BFP). At room temperature, the BFPs of Qa2 and H-2Db were 1.7 ± 0.2 and 0.6 ± 0.1 (micrometers ± SEM), respectively. Barriers persisted at 34°C, although the BFP for both MHC molecules was fivefold greater at 34°C than at 23°C. This indicates that barriers to lateral movement are primarily on the cytoplasmic half of the membrane and are dynamic.
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