We used the chimeric Arabidopsis cyclic nucleotide-gated ion channel AtCNGC11/12 to conduct a structurefunction study of plant cyclic nucleotide-gated ion channels (CNGCs). AtCNGC11/12 induces multiple pathogen resistance responses in the Arabidopsis mutant constitutive expresser of PR genes 22 (cpr22). A genetic screen for mutants that suppress cpr22-conferred phenotypes identified an intragenic mutant, #73, which has a glutamate to lysine substitution (E519K) at the beginning of the eighth b-sheet of the cyclic nucleotide-binding domain in AtCNGC11/12. The #73 mutant is morphologically identical to wild-type plants and has lost cpr22-related phenotypes including spontaneous cell death and enhanced pathogen resistance. Heterologous expression analysis using a K + -uptake-deficient yeast mutant revealed that this Glu519 is important for AtCNGC11/12 channel function, proving that the occurrence of cpr22 phenotypes requires active channel function of AtCNGC11/12. Additionally, Glu519 was also found to be important for the function of the wild-type channel AtCNGC12. Computational structural modeling and in vitro cAMP-binding assays suggest that Glu519 is a key residue for the structural stability of AtCNGCs and contributes to the interaction of the cyclic nucleotide-binding domain and the C-linker domain, rather than the binding of cAMP. Furthermore, a mutation in the a-subunit of the human cone receptor CNGA3 that causes total color blindness aligned well to the position of Glu519 in AtCNGC11/12. This suggests that AtCNGC11/12 suppressors could be a useful tool for discovering important residues not only for plant CNGCs but also for CNGCs in general.

The blood schizogonic cycle of human malaria parasites has thus far been the most exhaustively studied phase of parasite development. However, before entering red blood cells (RBCs), the parasite undergoes its first multiplication not in blood, but in hepatic cells. These hepatic stages were the last to be discovered and only a few studies have been performed in humans and other primates. Despite recent advances, in vivo studies have limitations and other approaches such as cultures of these liver forms may be necessary to investigate their chemosensitivity and their biochemical or immunological properties. Recently, sporozoites of species of rodent malaria have been made to infect cultured cell lines or primary hepatocyte cultures. We report here that the complete cycle of the human malaria parasite Plasmodium vivax can be obtained in primary cultures of human hepatocytes up to release of merozoites able to penetrate RBCs.

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