A member of the Plasmodium falciparum Pf60 multigene family codes for a nuclear protein expressed by readthrough of an internal stop codon

Four large multigene families have been described in Plasmodium falciparum malaria parasites (var, rif, stevor and Pf60). var and rif genes code for erythrocyte surface proteins and undergo clonal antigenic variation. We report here the characterization of the first Pf60 gene. The 6.1 gene is constitutively expressed by all mature blood stages and codes for a protein located within the nucleus. It has a single copy, 7-exon, 5' domain, separated by an internal stop codon from a 3' domain that presents a high homology with var exon II. Double-site immunoassay and P. falciparum transient transfection using the reporter luciferase gene demonstrated translation through the internal ochre codon. The 6.1 N-terminal domain has no homology with any protein described to date. Sequence analysis identified a leucine zipper and a putative nuclear localization signal and showed a high probability for coiled coils. Evidence for N-terminal coiled coil-mediated protein interactions was obtained. This identifies the 6.1 protein as a novel nuclear protein. These data show that the Pf60 and var genes form a superfamily with a common 3' domain, possibly involved in regulating homo- or heteromeric interactions.