Table 3. Summary of key residues and related observations.

Ubc9 Residue(s)	Function	Observations from Dynamics
Cys93	Catalytic cysteine [1], [4], [18].	Restricted in both Ubc9-SUMO and Ubc9-SUMO-RanBP2 complexes relative to unbound Ubc9 structure.
Val27 – Glu42	Includes loop between β-sheets serving as RanBP2 binding sites [20].	The loop has high mobility in unbound state. The mobility is further increased in Ubc9-SUMO complex. RanBP2 binding reduces mobility to unbound Ubc9 level.
		In Ubc9-SUMO complex, displays correlated fluctuations with RanBP2 binding regions of SUMO, which are spatially distant.
His83 – Ser89	Maintains hydrogen bonding networks around catalytic site, assists orientation of SUMO C-terminal Gly-Gly motif [4], [25], [27].	Correlated fluctuation with Asn121-Ala131 protected in Ubc9-SUMO and Ubc9-SUMO-RanBP2 complexes.
		Correlated fluctuation with Glu132-Arg141 are mostly lost in Ubc9-SUMO complex, but enhanced by RanBP2 binding.
Asp100 – Lys101	Target recognition [24], [25].	Increased msf in Ubc9-SUMO complex relative to unbound state. Fluctuations reduced below unbound state in Ubc9-SUMO-RanBP2 complex.
		Increased orientational freedom in Ubc9-SUMO complex, rigidification in Ubc9-SUMO-RanBP2 complex.
Asn124 – Pro128	Loop in contact with consensus sumoylation motif in targets [4].	Restricted in both Ubc9-SUMO and Ubc9-SUMO-RanBP2 complexes relative to unbound Ubc9 structure.
Glu132 – Arg 141	RanGAP1 specific target binding [4], [25], [27].	Orientational freedom of region reduced in Ubc9-SUMO-RanBP2 complex.
		Correlations with His83 – Ser89 are mostly lost in Ubc9-SUMO complex, but enhanced by RanBP2 binding.