Characterization of the membrane association of the influenza virus matrix protein in living cells

To determine if membrane association is an intrinsic properly of the influenza virus matrix protein (M₁) it was expressed from cDNA in living cells in the absence of other influenza virus proteins. By using a membrane fractionation scheme the M₁ protein was found to associate with membranes in a time-dependent manner (0 time = 45% total; after a 3-hr chase period = 68% total M₁ protein). Coexpression of the integral membrane proteins HA+NA+M₂ did not significantly increase the association of the M₁ protein with cellular membranes, indicating that putative interactions of the M₁ protein and the cytoplasmic tails of the integral membranes cannot be detected by this assay. Biochemical treatments of the M₁ protein associated with membranes with alkali, high salt conditions, or Triton X-114 yielded data that challenge the normal criteria for integral membrane proteins or peripheral membrane proteins. Examination of the solubility of the M₁ protein in influenza virus-infected cells to Triton X-100 extraction indicated it became increasingly insoluble with time, but the M, protein could be solubilized In Triton X-100 containing 1 M NaCl, suggesting an association of the M₁ protein with the cytoskeleton. However, when the M₁ protein was expressed from cDNA, it did not become insoluble to Triton X-100 extraction, suggesting an Interaction of the M₁ protein unique to the influenza virus-infected cell.