Molecular characterization of TgMIC5, a proteolytically processed antigen secreted from the micronemes of Toxoplasma gondii

During invasion of host cells, Toxoplasma gondii discharges the contents of small, apically located secretory organelles called micronemes. Micronemal proteins are known to be necessary for both parasite motility and invasion of host cells. To further define the contents of Toxoplasma micronemes, we used cell fractionation and secretion-modulating drugs to identify six nov el, putative micronemal proteins. In this paper we describe preliminary cha racterization of one of these novel proteins, TgMIC5. Molecular cloning and DNA sequence analysis of the TgMIC5 cDNA and gene revealed that it encodes a previously identified immunodominant antigen called H4. TgMIC5 also poss esses a consensus sequence unique to members of the parvulin family of pept idyl-prolyl cis-trans isomerases (PPIases). TgMIC5 is expressed as a prepro protein, which is proteolytically processed to a proprotein by signal pepti dase before being further processed to a mature protein of 22 kDa. Using a combination of protein secretion experiments, immunofluorescence and immuno electron microscopy, we demonstrated that TgMIC2 is stored in the microneme s of T. gondii tachyzoites before it is secreted into the surrounding mediu m. Based on its homology with parvulin-like PPIases, TgMIC5 may assist in t he folding of other micronemal proteins that function in invasion of host c ells by T. gondii tachyzoites. (C) 2000 Elsevier Science B.V. All rights re served.