Revision as of 05:26, 11 October 2018

SUMO (Small Ubiquitin-like Modifier)

Introduction

SUMOylation is a post-translational modification, which is involved in many cellular processes and provide a versatile way to regulate the dynamics of protein-protein interactions. Conjugation of the ubiquitin-related SUMO modifier to target proteins provides a platform for protein-protein interactions and ordered assembly of multi-protein complexes. In humans, three SUMO forms (SUMO1, SUMO2, and SUMO3) can be attached to lysine residues of target proteins. Modification of proteins by SUMO are recognized by SUMO-interacting motifs termed SIMs. In natural process, polySUMOylation recruits distinct interaction partners, such as E3 ubiquitin ligases, that bind to polySUMO chains through tandem SIMs. SIMs bind to a surface patch between the α-helix and a β-sheet of the SUMO protein and extend the β-sheet of SUMO by one additional strand. the SIM either attaches as a parallel or an antiparallel strand to the SUMO β-sheet. Binding is primarily mediated by a stretch of four residues containing 3–4 hydrophobic amino acids (I, V, or L). This core interaction motif is a common property of all SIMs.

Figure 1.Structure of a SUMO-SIM interaction

Design

Some membrane-less organelles, such as stress granules and P bodies, have been discovered in recent years. Proteins condense into droplets and assemble these organelles through a process called phase separation.

In order to rationally design a synthetic organelle based on protein phase separation, we need a multivalent module and a protein-protein interaction module. The paired SUMO and SIM is one of the bioparts that we chose to introduce protein-protein interaction. SUMO and SIM can dimerize spontaneously.

Figure 2.

Properties

Previous studies reveal that the electrostatic interactions can be modulated through PTMs on either SIM or SUMO, thereby regulating the dynamics and specificity of their interactions. Reversible phosphorylation/dephosphorylation of the serine or threonine residues in phosphoSIMs dramatically enhances the affinity of SUMO binding. Biophysical measurements of selected interactions revealed a phospho-dependent shift of the dissociation constant Kd from around 50 to 1.5 μM. The specific SUMO and SIM in our design were previously characterized in Michael K. Rosen’s lab. In their studies, the apparent polySUMO-polySIM dissociation constant Kd is 70–10,000 nM, estimated from ITC measurements with (SUMO)m + (SIM)m (m = 1, 2 or 3).

Our results confirm that SUMO/SIM module fused with HOTags can drive phase separation in yeast.

Figure12

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Sequence and Features