Development of Toxoplasma as a surrogate model.  The experimental challenges that arise out of apicomplexan parasite biology often make the pace of research frustratingly slow.  This no more true than in the study of the coccidian family where research over the last decade has languished.  The one exception is Toxoplasma gondii.  In this coccidian parasite experimental models have flourished.  Given their close relation, we wondered whether Toxoplasma could serve to break the deadlock by serving as a surrogate for other apicomplexan parasites (supported by USDA NRI 95-02064).  To this end, we attempted to express the Eimeria bovis refractile body protein, RB1.  RB1 is a major constituent of a prominent sporozoite body, and when expressed in T. gondii, RB1 also formed bodies with a similar appearance.  Mutagenesis of RB1 showed that aggregation in Toxoplasma is dependent on post-translational targeting and requires several conserved cysteine residues.  The new insights into RB1 biochemistry revealed similarities to plant protein storage bodies, and thus, these experiments have generated new ideas for this enigmatic structure in Eimeria.  We have now used Toxoplasma as a surrogate to express many other apicomplexan proteins from Plasmodium, Eimeria and Cryptosporidium.  Cryptosporidium is a human pathogen and like most coccidians is currently inaccessibly to genetic strategies.  Taking the surrogate route, we constructed Cryptosporidium genomic libraries for expression in Toxoplasma and successfully cloned the genes for C. parvum IMPDH and HXGPRT via genetic complementation in tachyzoites. (Striepen, White et al., PNAS, 2001).  Hence, where a reasonable selection strategy can be developed [e.g. secretory or surface antigens via flow cytometry; metabolic proteins by nutrient or drug selection] Toxoplasma can also be used as a surrogate to isolate apicomplexan genes by complementation.