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

To determine if membrane association is an intrinsic property of the influenza virus matrix protein (M(1)) it was expressed from cDNA in living cells in the absence of other influenza virus proteins. By using a membrane fractionation scheme the M(1) protein was found to associate with membranes in a time-dependent manner (0 time = 45% total; after a 3-hr chase period = 68% total M, protein). Coexpression of the integral membrane proteins HA+NA+M(2) did not significantly increase the association of the M, protein with cellular membranes, indicating that putative interactions of the M(1) protein and the cytoplasmic tails of the integral membranes cannot be detected by this assay. Biochemical treatments of the M(1) 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 M(1) protein in influenza virus-infected cells to Triton X-100 extraction indicated it became increasingly insoluble with time, but the M(1) protein could be solubilized in Triton X-100 containing 1 M NaCl, suggesting an association of the M(1) protein with the cytoskeleton. However, when the M(1) protein was expressed from cDNA, it did not become insoluble to Triton X-100 extraction, suggesting an interaction of the M(1) protein unique to the influenza virus-infected cell. (C) 1996 Academic Press, Inc.