Nutritional Stress Affects an Atypical Cap-Binding Protein in Leishmania

RNA Biol. 2012 Dec;9(12):1450-60. doi: 10.4161/rna.22709. Epub 2012 Nov 7.

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.

Keywords: 4E-binding peptide; Leishmania; RNA granules; eIF4E; eIF4G.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Cytoplasm / genetics
  • Cytoplasm / metabolism
  • Eukaryotic Initiation Factor-4E / genetics
  • Eukaryotic Initiation Factor-4E / metabolism*
  • Eukaryotic Initiation Factor-4G / genetics
  • Eukaryotic Initiation Factor-4G / metabolism
  • Leishmania / genetics
  • Leishmania / metabolism*
  • Microscopy, Confocal
  • Molecular Sequence Data
  • Multiprotein Complexes / genetics
  • Multiprotein Complexes / metabolism
  • Organisms, Genetically Modified / genetics
  • Organisms, Genetically Modified / metabolism
  • Protein Binding
  • Protein Interaction Mapping
  • Protein Transport
  • Protozoan Proteins / genetics
  • Protozoan Proteins / metabolism
  • RNA, Messenger / genetics
  • RNA, Messenger / metabolism
  • RNA, Protozoan / genetics
  • RNA, Protozoan / metabolism*
  • Stress, Physiological*
  • Two-Hybrid System Techniques

Substances

  • Eukaryotic Initiation Factor-4E
  • Eukaryotic Initiation Factor-4G
  • Multiprotein Complexes
  • Protozoan Proteins
  • RNA, Messenger
  • RNA, Protozoan