YihA GTPases localize to the apicoplast and mitochondrion of the malaria parasite and interact with LSU of organellar ribosomes

The YihA TRAFAC GTPases are critical for late-stage assembly of the ribosomal large subunit (LSU). In order to explore biogenesis of the reduced organellar ribosomes of the malaria parasite, we identified three nuclear-encoded homologs of YihA in Plasmodium falciparum. PfYihA1 targeted to the parasite apicoplast, PfYihA2 to the mitochondrion, and PfYihA3 was found in both the apicoplast and cytosol. The three PfYihA, expressed as recombinant proteins, were active GTPases and interacted with surrogate E. coli ribosomes in a nucleotide-independent manner. In vivo complexation of PfYihA with parasite organellar and/or cytosolic LSU was confirmed by co-immunoprecipitation using specific antibodies. Mitochondrial PfYihA2 carries a large C-ter extension with a strongly positively charged stretch. We hypothesise that this is important in compensating for the absence of helices of the central protuberance in the fragmented rRNA of Plasmodium mitoribosomes and may provide additional contact sites to aid in complex assembly. Combined with previous reports, our results indicate that P. falciparum mitochondria are likely to assemble ribosomes with the aid of PfEngA, PfObg1 and PfYihA2 GTPases while apicoplast ribosomes might use PfYihA1 and 3 in combination with other factors.