N-terminal targeting sequences and coding sequences act in concert to determine the localization and trafficking pathway of apicoplast proteins inToxoplasma gondii

Abstract

Toxoplasma gondiihas a relict plastid, the apicoplast; for most proteins that are localized in the apicoplast, N-terminal signal sequences precisely direct this localization. Proteins exclusively found in the apicoplast use a Golgi-independent route for trafficking, while dually targeted proteins found in both the apicoplast and the mitochondrion use a Golgi-dependent route. This report shows that the N-terminus of a dually targeted protein,TgTPx1/2, has all the signals for targeting both to the apicoplast and the mitochondrion yet exhibits flexibility in organellar targeting depending on the reporter protein. When enhanced Green Fluorescent Protein (eGFP) is used as a reporter,TgTPx1/2 targeting signals direct the protein to the mitochondrion, while the same targeting signals directT. gondiiendogenous proteinsTgSOD2 andTgSOD3 to the apicoplast. Chimeric proteins consisting of N-termini and coding sequences from different proteins were tested for both their apicoplast localization and their trafficking route to the apicoplast. This report shows, for the first time, that in addition to the N-terminal signal sequences, targeting and trafficking signals also reside within the coding sequences of apicoplast proteins. Our results hint that during evolution when the apicoplast endosymbiont was transferring its genes to the host nucleus, signal sequences such as those ofTgTPx1/2 enabled sampling of different sub-cellular compartments before the final destination of their protein products was fixed.