Peer Review #1 of "An ambiguous N-terminus drives the dual targeting of an antioxidant protein Thioredoxin peroxidase (TgTPx1/2) to endosymbiotic organelles in Toxoplasma gondii (v0.1)"

Abstract

Toxoplasma gondii harbors two endosymbiotic organelles: a relict plastid, the apicoplast and a mitochondrion. The parasite expresses an antioxidant protein, thioredoxin peroxidase 1/2 (TgTPx1/2), that is dually targeted to these organelles. Nuclear-encoded proteins such as TgTPx1/2 are trafficked to the apicoplast via a secretory route through the endoplasmic reticulum (ER) and to the mitochondrion via a non-secretory pathway comprising of translocon uptake. Given the two distinct trafficking pathways for localization to the two organelles, the signals in TgTPx1/2 for this dual targeting are open areas of investigation. Here we show that the signals for apicoplast and mitochondrial trafficking lie in the N-terminal 50 amino acids of the protein and are overlapping.Interestingly, mutational analysis of the overlapping stretch shows that despite this overlap, the signals for individual organellar uptake can be easily separated. Further, deletions in the N-terminus also reveal a 10 amino acid stretch that is responsible for targeting the protein from punctate structures surrounding the apicoplast into the organelle itself. Collectively, results presented in this report suggest that an ambiguous signal sequence for organellar uptake combined with a hierarchy of recognition by the protein trafficking machinery drives the dual targeting of TgTPx1/2. PeerJ reviewing PDF | Abstract 24 Toxoplasma gondii harbors two endosymbiotic organelles: a relict plastid, the apicoplast and a 25 mitochondrion. The parasite expresses an antioxidant protein, thioredoxin peroxidase 1/2 26 (TgTPx1/2), that is dually targeted to these organelles. Nuclear-encoded proteins such as 27 TgTPx1/2 are trafficked to the apicoplast via a secretory route through the endoplasmic 28 reticulum (ER) and to the mitochondrion via a non-secretory pathway comprising of translocon 29 uptake. Given the two distinct trafficking pathways for localization to the two organelles, the 30 signals in TgTPx1/2 for this dual targeting are open areas of investigation. Here we show that the 31 signals for apicoplast and mitochondrial trafficking lie in the N-terminal 50 amino acids of the 32 protein and are overlapping. Interestingly, mutational analysis of the overlapping stretch shows 33 that despite this overlap, the signals for individual organellar uptake can be easily separated. 34 Further, deletions in the N-terminus also reveal a 10 amino acid stretch that is responsible for 35 targeting the protein from punctate structures surrounding the apicoplast into the organelle itself. 36 Collectively, results presented in this report suggest that an ambiguous signal sequence for 37 organellar uptake combined with a hierarchy of recognition by the protein trafficking machinery 38 drives the dual targeting of TgTPx1/2. 39 40 Introduction 41 Endosymbiotic organelles, plastids and mitochondria, carry out specialized and essential 42 functions in eukaryotes. The proteins required for these functions are predominantly 43 nuclear-encoded and are trafficked from their site of s...