Assessing the components of the eIF3 complex and their phosphorylation status

Area of Science:

• Molecular Biology
• Protein Biochemistry

Background:

• The eukaryotic initiation factor 3 (eIF3) is a crucial, conserved multiprotein complex regulating translation initiation.
• Understanding eIF3's molecular functions requires detailed analysis of its composition and post-translational modifications.

Purpose of the Study:

• To investigate the composition and phosphorylation status of the eIF3 complex in Saccharomyces cerevisiae.
• To identify specific phosphorylation sites and the kinases involved in eIF3 modification.

Main Methods:

• Affinity purification and 2-D LC-MS/MS analysis of yeast eIF3 complexes.
• In vivo metabolic labeling with (32)P and in vitro kinase assays using casein kinase 2 (CK2).
• Immobilized metal affinity chromatography (IMAC) for phosphopeptide enrichment and tandem mass spectrometry.

Main Results:

• The yeast eIF3 complex comprises five core subunits and interacts with several other initiation factors and the CK2 complex.
• Nip1 was identified as a phosphoprotein, with CK2 phosphorylating three specific consensus sites (S98, S99, S103).
• Phosphorylation of Prt1 (S22, T707) and Tif5 (T191) was also detected; mutations of Nip1 phosphorylation sites resulted in slow growth.

Conclusions:

• CK2 directly phosphorylates Nip1 at specific sites, influencing eIF3 complex function.
• Phosphorylation of Nip1 by CK2 is critical for normal yeast growth.
• This study elucidates key regulatory mechanisms of translation initiation through eIF3 phosphorylation.