Autophagy-Related Atg8 Localizes to the Apicoplast of the Human Malaria Parasite Plasmodium falciparum

Kei Kitamura,Shigeharu Sato,Noboru Mizushima,Nobuo Ohta,Kiyoshi Kita,Chieko Kishi-Itakura,Takafumi Tsuboi,Tobias Spielmann

doi:10.1371/journal.pone.0042977

Abstract

Autophagy is a membrane-mediated degradation process, which is governed by sequential functions of Atg proteins. Although Atg proteins are highly conserved in eukaryotes, protozoa possess only a partial set of Atg proteins. Nonetheless, almost all protozoa have the complete factors belonging to the Atg8 conjugation system, namely, Atg3, Atg4, Atg7, and Atg8. Here, we report the biochemical properties and subcellular localization of the Atg8 protein of the human malaria parasite Plasmodium falciparum (PfAtg8). PfAtg8 is expressed during intra-erythrocytic development and associates with membranes likely as a lipid-conjugated form. Fluorescence microscopy and immunoelectron microscopy show that PfAtg8 localizes to the apicoplast, a four membrane-bound non-photosynthetic plastid. Autophagosome-like structures are not observed in the erythrocytic stages. These data suggest that, although Plasmodium parasites have lost most Atg proteins during evolution, they use the Atg8 conjugation system for the unique organelle, the apicoplast.

Highlights

Macroautophagy is a fundamental cellular process, by which cytoplasmic components including proteins and organelles are delivered to the lysosome for degradation
We found that P. falciparum Atg8 (PfAtg8) was associated with the apicoplast, not autophagosomes, during the erythrocytic stage
We report here that PfAtg8 is associated with the apicoplast, probably with the outermost membrane likely in a lipidconjugated form during the erythrocytic stage

Summary

Introduction

Macroautophagy ( referred to as autophagy hereafter) is a fundamental cellular process, by which cytoplasmic components including proteins and organelles are delivered to the lysosome (or vacuole in yeasts and plants) for degradation. Autophagy is involved in many cellular functions such as adaptation to starvation, cell differentiation, quality control of proteins and organelles, aging, and degradation of invading microbes [1,2,3,4,5,6]. It is implicated in human diseases such as cancer, inflammatory diseases, and neurodegeneration. The genetic hierarchy of these Atg proteins has been determined and they are classified into at least six functional groups: the starvationresponsive Atg kinase complex (Atg1–Atg13–Atg17–Atg29– Atg31), the multi-membrane spanning protein Atg, the class III phosphatidylinositol 3 (PtdIns 3)-kinase complex (Atg6–Atg14– Vps15–Vps34), the Atg2–Atg complex, the Atg12 – Atg5–Atg complex (‘‘–’’ denotes a covalent attachment), and the Atg8– phosphatidylethanolamine (PE) conjugate (Figure 1A) [8,9,10]

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Results

Conclusion