Phosphorylation of Toxoplasma gondii major surface antigens

The phosphorylation of the major surface proteins of Toxoplasma gondii tachyzoites was investigated. Metabolic labeling of intracellular tachyzoites with [32P]-orthophosphate followed by immunoprecipitation with specific monoclonal antibodies showed that all five major surface proteins were labeled as expected for GPI-anchored proteins. More detailed analysis of the phosphorylated sites revealed that surface proteins P30 and P22 contained phosphoserine residues in addition to the phosphorylated molecules that are presumably localized in the GPI-membrane anchors.