Measles virus matrix protein is not cotransported with the viral glycoproteins but requires virus infection for efficient surface targeting

As we have shown earlier, the measles virus (MV) glycoproteins H and F are expressed on both, the apical and the basolateral membrane of polarized Madin–Darby canine kidney cells. In contrast to the glycoproteins, we found the viral matrix protein (M) to accumulate selectively at the apical plasma membrane of MV-infected cells. M did not colocalize with the glycoproteins at basolateral membranes of polarized cells indicating an independent surface transport mechanism. Analysis of infected cells treated with monensin supported this view. When H and F were retained in the medial Golgi by monensin treatment, M did not accumulate in this cellular compartment. To elucidate the subcellular transport mechanism of the cytosolic M protein, M was expressed in the absence of other viral proteins. Flotation analysis demonstrated that most of the M protein coflotated in infected or in M-transfected cells with cellular membranes. Thus, the M protein possesses the intrinsic ability to bind to lipid membranes. Unexpectedly, plasmid-encoded M protein was rarely found to accumulate at surface membranes. Although cotransport with the viral glycoproteins was not needed, M transport to the plasma membrane required a component only provided in MV-infected cells.