AvrRxv is a member of a family of pathogen effectors present in pathogens of both plant and mammalian species. Xanthomonas campestris pv. vesicatoria strains carrying AvrRxv induce a hypersensitive response (HR) in the tomato cultivar Hawaii 7998. Using a yeast two-hybrid screen, we identified a 14-3-3 protein from tomato that interacts with AvrRxv called AvrRxv Interactor 1 (ARI1). The interaction was confirmed in vitro with affinity chromatography. Using mutagenesis, we identified a 14-3-3-binding domain in AvrRxv and demonstrated that a mutant in that domain showed concomitant loss of interaction with ARI1 and HR-inducing activity in tomato. These results demonstrate that the AvrRxv bacterial effector recruits 14-3-3 proteins for its function within host cells. AvrRxv homologues YopP and YopJ from Yersinia do not have AvrRxv-specific HR-inducing activity when delivered into tomato host cells by Agrobacterium. Although YopP itself cannot induce HR, its C-terminal domain containing the catalytic residues can replace that of AvrRxv in an AvrRxv-YopP chimera for HR-inducing activity. Phylogenetic analysis indicates that the sequences encoding the C-termini of family members are evolving independently from those encoding the N-termini. Our results support a model in which there are three functional domains in proteins of the family, translocation, interaction, and catalytic.

A repetitive region of Plasmodium berghei merozoite surface protein-1 (PbMSP-1) was expressed as a fusion protein with either maltose binding protein or the B subunit of heat-labile enterotoxin from Escherichia coli. Vaccination of mice with the fusion proteins indicates that this region of PbMSP-1 is antigenic as evidenced by an antibody response. The fusion proteins were also expressed in Salmonella and mice were orally immunized with the recombinant Salmonella. Some of the vaccinated mice survived a challenge with P. berghei blood-stage parasites without developing parasitemia. All control mice became patent and succumbed to the challenge infection. This partial protection was also observed with purified recombinant protein and was independent of the adjuvant used. Mice immunized with recombinant Salmonella showed either extremely low or no antibody response to PbMSP-1, suggesting that cell-mediated immunity is important for the observed protection. These studies show that it is feasible to develop a cost effective oral vaccine against the blood stage of the malarial parasite.

The complete gene for merozoite surface protein-1 (MSP-1) from Plasmodium berghei has been cloned and sequenced. Comparison of the P. berghei MSP-1 sequence with MSP-1 from other rodent parasites reveals five conserved domains interrupted by four variable blocks. These variable blocks exhibit no sequence homology but do have similar amino acid compositions. Primary proteolytic processing sites are located near the boundaries between the conserved domains and the variable blocks. Sequencing of the variable blocks from several P. berghei isolates shows that the predominant intra-species difference is in the number of tandem repeats. The inter-and intra-species differences suggest that the variable blocks are localized areas with relatively high levels of slipped-strand mispairing, unequal crossing-over, or other intragenic recombination activity. MSP-1 from P. berghei exhibits more repetitiveness than MSP-1 from other species suggesting that P. berghei experiences a higher intrinsic level of events producing variable numbers of tandem repeats or a lower level of events leading to the degeneration of tandem repeats.