Kinase-associated Endopeptidase 1 (Kae1) Participates in an Atypical Ribosome-associated Complex in the Apicoplast of Plasmodium falciparum

Background: Kae1 proteins are universally conserved and are regulated by a variety of protein-protein interactions. Results:P. falciparum expresses two Kae1 proteins with distinct localizations and atypical binding partners. Conclusion: Kae1(api) is essential and a potential regulator of ribosome activity. Significance: The Kae1 protein family likely has an expanded role outside of its known tRNA-modifying activity. The universally conserved kinase-associated endopeptidase 1 (Kae1) protein family has established roles in N-6-threonylcarbamoyl adenosine tRNA modification, transcriptional regulation, and telomere homeostasis. These functions are performed in complex with a conserved core of protein binding partners. Herein we describe the localization, essentiality, and protein-protein interactions for Kae1 in the human malaria parasite Plasmodium falciparum. We found that the parasite expresses one Kae1 protein in the cytoplasm and a second protein in the apicoplast, a chloroplast remnant organelle involved in fatty acid, heme, and isoprenoid biosynthesis. To analyze the protein interaction networks for both Kae1 pathways, we developed a new proteomic cross-validation approach. This strategy compares immunoprecipitation-MS data sets across different cellular compartments to enrich for biologically relevant protein interactions. Our results show that cytoplasmic Kae1 forms a complex with Bud32 and Cgi121 as in other organisms, whereas apicoplast Kae1 makes novel interactions with multiple proteins in the apicoplast. Quantitative RT-PCR and immunoprecipitation studies indicate that apicoplast Kae1 and its partners interact specifically with the apicoplast ribosomes and with proteins involved in ribosome function. Together, these data indicate an expanded, apicoplast-specific role for Kae1 in the parasite.