496–505 Nucleic Acids Research, 2006, Vol. 34, No. 2 doi:10.1093/nar/gkj450 The Plasmodium selenoproteome Alexey V. Lobanov 1 , Cesar Delgado1,2, Stefan Rahlfs3, Sergey V. Novoselov 1 , Gregory V. Kryukov 1 , Stephan Gromer4, Dolph L. Hatfield5, Katja Becker3 and Vadim N. Gladyshev1,* 1Department of Biochemistry and 2Department of Computer Science, University of Nebraska, Lincoln, NE 68588, USA, 3Interdisziplina¨ res Forschungszentrum, Justus-Liebig University Giessen, Heinrich-Buff-Ring 26-32, D-35392 Giessen, Germany, 4Biochemie-Zentrum der Universita¨ t Heidelberg, Im Neuenheimer Feld 504, D-69120 Heidelberg, Germany and 5Molecular Biology of Selenium Section, Laboratory of Cancer Prevention, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, USA Received August 22, 2005; Revised October 3, 2005; Accepted January 3, 2006 ABSTRACT The use of selenocysteine (Sec) as the 21st amino acid in the genetic code has been described in all three major domains of life. However, within eukary- otes, selenoproteins are only known in animals and algae. In this study, we characterized selenopro- teomes and Sec insertion systems in protozoan Apicomplexa parasites. We found that among these organisms, Plasmodium and Toxoplasma utilized Sec, whereas Cryptosporidium did not. However, Plasmodium had no homologs of known selenopro- teins. By searching computationally for evolution- arily conserved selenocysteine insertion sequence (SECIS) elements, which are RNA structures involved in Sec insertion, we identified four unique Plasmod- ium falciparum selenoprotein genes. These seleno- proteins were incorrectly annotated in PlasmoDB, were conserved in other Plasmodia and had no detectable homologs in other species. We provide evidence that two Plasmodium SECIS elements sup- ported Sec insertion into parasite and endogenous selenoproteins when they were expressed in mamma- lian cells, demonstrating that the Plasmodium SECIS elements are functional and indicating conservation of Sec insertion between Apicomplexa and animals. Dependence of the plasmodial parasites on selenium suggests possible strategies for antimalarial drug development. INTRODUCTION Plasmodium falciparum, the organism which causes tropical malaria in man, has a genome of 23 Mb, the entire sequence of which has been determined (1). Signiﬁcant sequence information is also available for several other plasmodial species (2), as well as for related apicomplexan pathogens, including Toxoplasma and Cryptosporidium species (3–5). These genomics resources stimulated research on these patho- genic protozoa (6,7). However, much remains unknown about the biology of these parasites. Selenium is a trace element with signiﬁcant biomedical potential. It is inserted into protein in response to a UGA codon as selenocysteine (Sec), the 21st amino acid in the genetic code (8–11). However, for most organisms, their dependence on selenium and selenoproteins is not known. Initial sequence and database analyses have shown the presence of a putative Sec-speciﬁc elongation factor EFSec/eSelB in P.falciparum, suggesting the possibility that Plasmodium utilizes Sec insertion and contains Sec-containing proteins. Known selenoproteins have Sec in active sites to carry out redox functions (10). Selenoproteins are highly reactive compared with their cysteine-containing analogs, but are also susceptible to inac- tivation by a variety of electrophilic agents. In particular, the use of gold compounds and alkylating agents allows speciﬁc tar- geting of Sec residues in proteins (12,13). Thus, the presence of selenoproteins may suggest novel therapeutic strategies. The identiﬁcation of selenoproteins in plasmodial species is also of interest from an evolutionary point of view. Seleno- proteomes have been characterized in the three major domains of life (14–18). Within prokaryotes, for which complete genomic sequences are available, selenoproteins have been described in several archaea and in diverse bacteria species. However, within eukaryotes, selenoproteins are so far only known in animals and algae (19,20). In fact, a large protozoan community, including Apicomplexa, has been essentially unexplored with regard to the role of the Sec insertion in these organisms. Recently, tRNASec was reported in Plasmodia and a computational search for selenocysteine insertion sequence *To whom correspondence should be addressed. Tel: +1 402 472 4948; Fax: +1 402 472 7842; Email: vgladyshev1@unl.edu The Author 2006. Published by Oxford University Press. All rights reserved. The online version of this article has been published under an open access model. 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