Endosomal association of a protein phosphatase with high dephosphorylating activity against a coronavirus nucleocapsid protein

On the assumption that dephosphorylation of the neurotropic coronavirus JHM (JHMV) nucleocapsid protein (N) may be connected with initiation of the infectious cycle we searched for a relevant host enzyme activity. Analysis of subcellular fractions from L‐2 murine fibroblasts, separated by dual Percoll density gradients, revealed the presence of a phosphoprotein phosphatase (PPPase), co‐sedimenting with the endososomal/prelysosomal material, which possesses high activity against N. With purified [(22)P]N as substrate it was demonstrated that this PPPase, distinguishable from acid and alkaline phosphatases, acts optimally at neutral pH in the presence of Mn(2+) following treatment with a detergent. Complete inhibition with okadaic acid at 0.9–4.5 μM but not at 1–10 nM relegates this PPase to a type I protein phosphatase. Similar PPPase activity for N was present in the endosome fraction of a rat Roc‐1 astrocytoma‐oligodendrocyte cell line and in homogenates of brain and cultured oligodendrocytes. Our data suggest that the phosphorylated N of the inoculum may be modified by the endosomal PPPase in host cells, including those from the CNS so as to facilitate the JHMV infectious process.