title: Nutritional stress affects an atypical cap-binding protein in Leishmania.
abstract: Many eukaryotes encode multiple isoforms of the cap-binding translation initiation factor (eIF4E). Leishmanias and other trypanosomatids encode four paralogs of this protein, but none can complement the eIF4E function in a yeast mutant. A low conservation is observed between the four paralogs, suggesting they assist these organisms survive a multitude of conditions encountered throughout the life cycle. Earlier attempts to decipher their function led to identification of LeishIF4E-4 as the canonical translation initiation factor. LeishIF4E-1 appears to function during thermal stress, via a mechanism not yet understood. LeishIF4E-3 hardly binds cap-4 and is, therefore, less likely to serve as a typical initiation factor. Although it interacts with an eIF4G homolog, LeishIF4G-4, the two polypeptides do not co-migrate on sucrose gradients. While LeishIF4E-3 enters large particles that increase in size during nutritional stress, LeishIF4G-4 is found only in the top fractions. Confocal microscopy localized LeishIF4E-3 (but not LeishIF4G-4) within nutritional stress-induced granules. Accordingly, interaction between the two proteins reduced upon starvation. We therefore propose that under normal conditions, LeishIF4G-4 sequesters LeishIF4E-3 in the cytoplasm. During a nutritional stress, LeishIF4E-3 is modified and released from LeishIF4G-4 to enter stress granules, where inactive mRNAs are stored. Binding of LeishIF4G-4 to LeishIF4E-3 requires a short peptide within the LeishIF4G-4 N-terminus, which bears no similarity to the consensus 4E-binding peptide, YXXXXLΦ. Mutational analysis combined with structure prediction indicates that this interaction is based on an obligatory, conserved α helix in LeishIF4G-4. These features further highlight the uniqueness of LeishIF4E-3 and how it interacts with its binding partners.
authors.affiliation: Department of Life Sciences; Ben-Gurion University of the Negev; Beer Sheva, Israel.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Zinoviev
authors.forename: Shachar
authors.initials: S
authors.lastname: Manor
authors.forename: Michal
authors.initials: M
authors.lastname: Shapira
mesh_terms: D000595:Amino Acid Sequence; D003593:Cytoplasm / Q000235:genetics / Q000378:metabolism; D039561:Eukaryotic Initiation Factor-4E / Q000235:genetics / Q000378:metabolism*; D039603:Eukaryotic Initiation Factor-4G / Q000235:genetics / Q000378:metabolism; D007891:Leishmania / Q000235:genetics / Q000378:metabolism*; D018613:Microscopy, Confocal; D008969:Molecular Sequence Data; D046912:Multiprotein Complexes / Q000235:genetics / Q000378:metabolism; D030781:Organisms, Genetically Modified / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D025941:Protein Interaction Mapping; D021381:Protein Transport; D015800:Protozoan Proteins / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D016053:RNA, Protozoan / Q000235:genetics / Q000378:metabolism*; D013312:Stress, Physiological; D020798:Two-Hybrid System Techniques
chemical_list: D039561:Eukaryotic Initiation Factor-4E; D039603:Eukaryotic Initiation Factor-4G; D046912:Multiprotein Complexes; D015800:Protozoan Proteins; D012333:RNA, Messenger; D016053:RNA, Protozoan
keywords: 4E-binding peptide; Leishmania; RNA granules; eIF4E; eIF4G