Fig. 3. Transport pathways in polarised epithelial MDCK cells and T lymphocytes. (A) In MDCK epithelial cells, newly synthesised proteins are segregated after passage through the Golgi in different vesicular carriers destined for the apical (red) and basolateral (green) subdomains, which have different protein compositions and functions. Partitioning of proteins into rafts appears to mediate the sorting of at least some apical membrane proteins, such as HA, whereas basolateral sorting (green arrow) is dependent on the existence of a specific signal in the cytoplasmic tail of membrane proteins. Caveolae are raft-containing invaginated structures exclusively located in the basolateral surface. MAL and caveolin-1 are machinery involved in raft-dependent apical transport (straight arrow in red) and caveolae formation (curved arrow in red), respectively. (B) Polarised migrating T lymphocytes display two poles: the leading edge at the front and a membrane protrusion (the uropod) at the trailing edge, each of which has a specific protein composition and function. HA appears to employ rafts for biosynthetic transport (red arrow) to the uropod, which contains rafts. T cells lack caveolin-1 but do express MAL. (C) Caveolin-1 is necessary for caveolae formation and organises lipid rafts to build the caveolar architecture. (D) MAL is necessary for apical transport and appears to organise lipid rafts for the formation of the transport vesicles.