The MAL proteolipid is necessary for normal apical transport and accurate sorting of the influenza virus hemagglutinin in Madin-Darby canine kidney cells

The MAL (MAL/VIP17) proteolipid is a nonglycosylated integral membrane prot ein expressed in a restricted pattern of cell types, including T lymphocyte s, myelin-forming cells, and polarized epithelial cells. Transport of the i nfluenza virus hemagglutinin (HA) to the apical surface of epithelial Madin -Darby canine kidney (MDCK) cells appears to be mediated by a pathway invol ving glycolipid- and cholesterol-enriched membranes (GEMs). In MDCK cells, MAL has been proposed previously as being an element of the protein machine ry for the GEM-dependent apical transport pathway Using an antisense oligon ucleotide-based strategy and a newly generated monoclonal antibody to canin e MAL, herein we have approached the effect of MAL depletion on HA transpor t in MDCK cells. We have found that MAL depletion diminishes the presence o f HA in GEMs, reduces the rate of HA transport to the cell surface, inhibit s the delivery of HA to the apical surface, and produces partial missorting of HA to the basolateral membrane. These effects were corrected by ectopic expression of MAL in MDCK cells whose endogenous MAL protein was depleted. Our results indicate that MAL is necessary for both normal apical transpor t and accurate sorting of HA.