Analysis of Euglena gracilis Plastid-Targeted Proteins Reveals Different Classes of Transit Sequences

Department of Biology, University of New Brunswick, Fredericton, NB, Canada, E3B 5A3; Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, NS, Canada, B3H 1X5

^* To whom correspondence should be addressed. Email: durnford{at}unb.ca,

	Abstract

The plastid of Euglena gracilis was acquired secondarily through an endosymbiotic event with a eukaryotic green alga and, as a result, is surrounded by a third membrane. This membrane complexity raises the question of how the plastid proteins are targeted to and imported into the organelle. To further explore plastid protein targeting in Euglena, we screened a total of 9461 EST clusters (derived from 19,013 individual ESTs) for full-length proteins that are plastid-localized, to characterize their targeting sequences and to infer potential modes of translocation. Of the 117 proteins identified as being potentially plastid-localized and whose N-terminal targeting sequence could be inferred, 83 were unique and could be classified into two major groups. Class I proteins have tripartite targeting sequences, comprising (in order) an N-terminal signal sequence, a plastid transit peptide (TP) domain, and a predicted stop-transfer sequence. Within this class of proteins are the lumen-targeted proteins (Class IB) having an additional hydrophobic domain similar to a signal sequence and required for further targeting across the thylakoid membrane. Class II proteins lack the putative stop-transfer sequence and possess only a signal sequence at the N-terminus, followed by what in amino acid composition resembles a plastid transit peptide. Unexpectedly, a few unrelated plastid-targeted proteins exhibit highly similar transit sequences, implying either a recent swapping of these domains or a conserved function. This work represents the most comprehensive description to date of transit peptides in Euglena and hints at the complex routes of plastid targeting that must exist in this organism.