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Abstract
Genome sequencing revealed an extreme AT-rich genome and a profusion of asparagine repeats associated with low complexity regions (LCRs) in proteins of the malarial parasite Plasmodium falciparum. Despite their abundance, the function of these LCRs remains unclear. Because they occur in almost all families of plasmodial proteins, the occurrence of LCRs cannot be associated with any specific metabolic pathway; yet their accumulation must have given selective advantages to the parasite. Translation of these asparagine-rich LCRs demands extraordinarily high amounts of asparaginylated tRNA(Asn). However, unlike other organisms, Plasmodium codon bias is not correlated to tRNA gene copy number. Here, we studied tRNA(Asn) accumulation as well as the catalytic capacities of the asparaginyl-tRNA synthetase of the parasite in vitro. We observed that asparaginylation in this parasite can be considered standard, which is expected to limit the availability of asparaginylated tRNA(Asn) in the cell and, in turn, slow down the ribosomal translation rate when decoding asparagine repeats. This observation strengthens our earlier hypothesis considering that asparagine rich sequences act as "tRNA sponges" and help cotranslational folding of parasite proteins. However, it also raises many questions about the mechanistic aspects of the synthesis of asparagine repeats and about their implications in the global control of protein expression throughout Plasmodium life cycle.
Highlights
Proteins from the malaria parasite Plasmodium contain extensive asparagine repeats
Plasmodia asparaginyl-tRNA synthetase (AsnRS) were compared with homologues from H. sapiens, the vertebrate host of P. falciparum, and from Brugia malayi, for comparison with the crystal structure determined by Crepin and collaborators [27]
Impact on Protein Synthesis versus a Specific Function for Asparagine-rich low complexity regions (LCRs)—Single amino acid repeats have been found for all 20 amino acids in all three domains of life
Summary
Proteins from the malaria parasite Plasmodium contain extensive asparagine repeats. Results: Plasmodium tRNAAsn concentration and in vitro asparaginylation catalytic parameters were analyzed to explain the frequency of asparagine repeats. Because they occur in almost all families of plasmodial proteins, the occurrence of LCRs cannot be associated with any specific metabolic pathway; yet their accumulation must have given selective advantages to the parasite Translation of these asparagine-rich LCRs demands extraordinarily high amounts of asparaginylated tRNAAsn. unlike other organisms, Plasmodium codon bias is not correlated to tRNA gene copy number. We observed that asparaginylation in this parasite can be considered standard, which is expected to limit the availability of asparaginylated tRNAAsn in the cell and, in turn, slow down the ribosomal translation rate when decoding asparagine repeats This observation strengthens our earlier hypothesis considering that asparagine rich sequences act as “tRNA sponges” and help cotranslational folding of parasite proteins. It raises many questions about the mechanistic aspects of the synthesis of asparagine repeats and about their implications in the global control of protein expression throughout Plasmodium life cycle
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