Membrane activity of the southern cowpea mosaic virus coat protein: the role of basic amino acids, helix-forming potential, and lipid composition.

Southern cowpea mosaic virus (SCPMV) is a spherical RNA virus with T = 3 icosahedral symmetry. The particle is composed of 180 subunits of the coat protein (CP) and one copy of the positive-sense viral RNA. The CP has two domains, the random (R) domain formed by the N-terminal 64 aa and the shell (S) domain (aa 65--260). The R domain is highly charged, with 11 of the N-terminal 30 residues being basic. It is localized to the interior of the native particle where it may interact with the viral RNA, but under certain pH and salt conditions the topology of the particle changes to externalize the R domain. Since the CPs of several spherical RNA viruses have been shown to interact with host membranes during infection, we have begun investigating the membrane interactions of the SCPMV CP using the artificial liposome membranes. Both the native CP and the R domain overexpressed in Escherichia coli were observed to interact with liposomes. The interaction between the R domain and liposomes required either anionic phospholipids or non-bilayer-forming lipids and involved electrostatic interactions since it was shown to be both pH and ionic strength dependent. The analysis of four different deletion and six different site-directed substitution mutations partially mapped the region responsible for this interaction to residues 1--30. Analysis of this region of the R domain by circular dichroism indicated that it assumes an alpha-helical structure when exposed to liposomes composed of anionic lipids. Mutations, which extend the helical nature of this region, promoted an increased interaction. The possible role of the CP/lipid interaction in the SCPMV infection is discussed.