Focal junctions retard lateral movement and disrupt fluid phase connectivity in the plasma membrane

In cultured keratinocytes, focal junctions are enriched in major constituents of lipid rafts, such as GM1 ganglioside, phosphoinositides, caveolins and flotillins. We have therefore speculated that focal junctions represent superrafts formed by coalescence of microdomains into large areas containing liquid-ordered (Lo) lipids. Indeed, values of maximal fluorescence recovery after photobleaching revealed that the long-range mobility of cholera toxin B subunit (CTB, marker of Lo) was ∼1.5-fold retarded within the focal junctions compared to the surrounding membrane. However, 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiI-C18:0), which specifically partitions to the liquid-disordered (Ld), non-raft phase, was also enriched in focal junctions and its mobility was slightly retarded. Cross-linking of GM1 by CTB or raft aggregation by methyl-β-cyclodextrin further decreased the recovery of DiI-C18:0. We propose a model in which focal junctions impose lateral heterogeneity in the plasma membrane by entrapment of lipid microdomains between dense arrays of immobilized transmembrane molecules which can enmesh otherwise freely percolating Ld phase lipids.