ALBERT

Description: The internal compartmentation of eukaryotic cells not only allows separation of biochemical processes but it also creates the requirement for systems that can selectively transport proteins across the membrane boundaries. Although most proteins function in a single subcellular compartment, many are able to enter two or more compartments, a phenomenon known as dual or multiple targeting. The aminoacyl-tRNA synthetases (aaRSs), which catalyze the ligation of tRNAs to their cognate amino acids, are particularly prone to functioning in multiple subcellular compartments. They are essential for translation, so they are required in every compartment where translation takes place. In diatoms, there are three such compartments, the plastid, the mitochondrion, and the cytosol. In cryptophytes, translation also takes place in the periplastid compartment (PPC), which is the reduced cytoplasm of the plastid’s red algal ancestor and which retains a reduced red algal nucleus. We searched the organelle and nuclear genomes of the cryptophyte Guillardia theta and the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana for aaRS genes and found an insufficient number of genes to provide each compartment with a complete set of aaRSs. We therefore inferred, with support from localization predictions, that many aaRSs are dual targeted. We tested four of the predicted dual targeted aaRSs with green fluorescent protein fusion localizations in P. tricornutum and found evidence for dual targeting to the mitochondrion and plastid in P. tricornutum and G. theta , and indications for dual targeting to the PPC and cytosol in G. theta. This is the first report of dual targeting in diatoms or cryptophytes.

Description: The establishment of a metabolic connection between host and symbiont is a crucial step in the evolution of an obligate endosymbiotic relationship. Such was the case in the evolution of mitochondria and plastids. Whereas the mechanisms of metabolite shuttling between the plastid and host cytosol are relatively well studied in Archaeplastida—organisms that acquired photosynthesis through primary endosymbiosis—little is known about this process in organisms with complex plastids. Here, we focus on the presence, localization, and phylogeny of putative triose phosphate translocators (TPTs) in the complex plastid of diatoms. These proteins are thought to play an essential role in connecting endosymbiont and host metabolism via transport of carbohydrates generated by the photosynthesis machinery of the endosymbiont. We show that the complex plastid localized TPTs are monophyletic and present a model for how the initial metabolic link between host and endosymbiont might have been established in diatoms and other algae with complex red plastids and discuss its implications on the evolution of those lineages.