The S. pombe translation initiation factor eIF4G is Sumoylated and associates with the SUMO protease Ulp2.

Jirapas Jongjitwimol,Simon J Morley,Lauren Small,Deborah L Taylor,Lihong Zhou,Felicity Z Watts,Robert Baldock,Duncan Smith,Min Feng,Oliver Wilkinson,Lucas D Bowler

doi:10.1371/journal.pone.0094182

Abstract

SUMO is a small post-translational modifier, that is attached to lysine residues in target proteins. It acts by altering protein-protein interactions, protein localisation and protein activity. SUMO chains can also act as substrates for ubiquitination, resulting in proteasome-mediated degradation of the target protein. SUMO is removed from target proteins by one of a number of specific proteases. The processes of sumoylation and desumoylation have well documented roles in DNA metabolism and in the maintenance of chromatin structure. To further analyse the role of this modification, we have purified protein complexes containing the S. pombe SUMO protease, Ulp2. These complexes contain proteins required for ribosome biogenesis, RNA stability and protein synthesis. Here we have focussed on two translation initiation factors that we identified as co-purifying with Ulp2, eIF4G and eIF3h. We demonstrate that eIF4G, but not eIF3h, is sumoylated. This modification is increased under conditions that produce cytoplasmic stress granules. Consistent with this we observe partial co-localisation of eIF4G and SUMO in stressed cells. Using HeLa cells, we demonstrate that human eIF4GI is also sumoylated; in vitro studies indicate that human eIF4GI is modified on K1368 and K1588, that are located in the C-terminal eIF4A- and Mnk-binding sites respectively.

Highlights

Sumoylation is a post-translational protein modification that is required for numerous processes within cells, including transcription, chromosome segregation, DNA damage responses, cell signalling and meiosis
Since S. pombe Ulp2 more closely resembles S. cerevisiae Ulp2 than it does Ulp1, it is likely that the main activity of S. pombe Ulp2 is in deconjugating SUMO from sumoylated targets rather than in processing SUMO to the mature form
A number of proteins required for ribosome biogenesis, including some of those we identified by mass spectrometry, have recently been demonstrated to be sumoylated (Table 2) [6,7,23

Summary

Introduction

Sumoylation is a post-translational protein modification that is required for numerous processes within cells, including transcription, chromosome segregation, DNA damage responses, cell signalling and meiosis (reviewed in [1,2,3,4,5,6,7]). SUMO is a small ubiquitin-like modifier that is attached to lysine residues in target proteins. Processing of SUMO requires a specific SUMO-protease [12,13,14], and involves the removal of a small number of amino acids from the C-terminus of precursor SUMO to reveal a GlyGly motif. From here SUMO is passed to the SUMO conjugating enzyme (E2), where it again forms a thioester bond with another cysteine residue. SUMO can be attached to one or more lysine residues in the target protein. One of a small number of SUMO ligases (E3) is required for conjugation. The removal of SUMO from target proteins or dismantling of SUMO chains occurs via the action of SUMO-specific proteases [14,15]

Methods

Results

Conclusion