Lymphocytic choriomeningitis virus. IX. Properties of the nucleocapsid

Nucleocapsids (NC) of lymphocytic choriomeningitis (LCM) virus were obtained by treatment of purified infectious virus with detergent and salt at high concentrations, followed by gradient centrifugation. NC thus prepared contained 31 S RNA, 23 S RNA, and a protein with an apparent mol wt 63,000 and an isoelectric point pH 5.7 (p63), assumed to be the nucleoprotein. We had previously observed that the intact LCM virus contained two kinds of protein with mol wt 63,000 separable by their isoelectric points which were pH 5.7 and 7.7, respectively. We now found that the latter component was removed from the NC together with the glycoproteins, and labeling studies revealed that it was closely associated with the viral envelope. We have named the protein with a mol wt of 63,000 and an isoelectric point pH 7.7 p63E (E for envelope). By limited proteolysis both viral components of mol wt 63,000 could not be distinguished, indicating that they are basically identical. It appears that p63E is the phosphorylated form of p63.