Affiliation:
1. Department of Biology, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3
2. Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 1X5
Abstract
ABSTRACTThe plastid ofEuglena graciliswas acquired secondarily through an endosymbiotic event with a eukaryotic green alga, and as a result, it 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 inEuglena, we screened a total of 9,461 expressed sequence tag (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 whose N-terminal targeting sequences 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 domain, and a predicted stop-transfer sequence. Within this class of proteins are the lumen-targeted proteins (class IB), which have 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 inEuglenaand hints at the complex routes of plastid targeting that must exist in this organism.
Publisher
American Society for Microbiology
Subject
Molecular Biology,General Medicine,Microbiology
Cited by
70 articles.
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