An unexpected effect of divalent cations on the adenovirus endogenous protein kinase: Alteration in the specificity of phosphorylation

Abstract

The adenovirus endogenous protein kinase specifically phosphorylates capsid protein IIIa in the presence of Mg 2+, utilizing either ATP or GTP as a phosphate donor. When Mn 2+ is substituted for Mg 2+, in the presence of ATP, phosphorylation of IIIa is enhanced by 2–3 fold. However, in addition to IIIa phosphorylation, the core proteins V and VII are now phosphorylated. A similar finding is made when Co 2+ is used instead of Mg 2+. Further, when Mn 2+ or Co 2+ is substituted for Mg 2+, the phosphoamino acid residue profile is changed, viz., instead of only phosphoserine being labeled, phosphothreonine is now extensively labeled. These results are specific for the above endogenous viral proteins, since when the viral protein kinase is used to phosphorylate added exogenous substrates such as casein, no enhancement of phosphorylation nor any change in phosphoamino acid profile is achieved by substituting Mn 2+ for Mg 2+. It is thus likely that MnATP or CoATP somehow interacts differently with adenovirus structural proteins than does MgATP and this facilitates their accessibility to the enzyme.