Membrane and Phospholipid Binding by Murine Coronaviral Nucleocapsid N Protein

Evidence is presented which indicates that membrane binding of the murine coronavirus, mouse hepatitis virus (MHV) nucleocapsid (N) protein is mediated by certain lipids. Binding of N protein to membranes of mouse fibroblast L-2 cells is very specific and occurs under conditions in which no other viral or cellular proteins show datectable binding. Binding occurs with both free and nucleocapsid-associated N protein, arguing for membrane-binding sites on the N protein itself, rather than on RNA. Binding of N protein also occurs to membranes in the absence of MHV matrix (M) protein which is known to interact with the N protein. Both non-viral, single-stranded RNA and DNA inhibit membrane binding of N protein. In addition, purified phospholipid liposomes compete against N protein binding to membranes. Of various phospholipids tested, cardiolipin was the most effective in inhibiting membrane binding. The N protein was also shown to bind directly to phospholipid liposomes containing cardiolipin as well as to liposomes containing total lipids extracted from mouse L-2 cells. Because of certain structural similarities between phospholipids and nucleic acids, we speculate that membrane lipid association of the N protein may compete for RNA binding sites on the N protein. Such a mechanism may be important for processes such as nucleocapsid uncoating and nucleocapsid assembly. Most interestingly the properties of phospholipid and nucleic acid binding are markedly similar to those recently described for the bacterial DNA-binding proteins DnaA and recA.