An initially counter-intuitive observation regarding SUMOylation is that only small proportion of the available substrate protein need be SUMOylated to achieve maximal effect. This phenomenon has been referred to as the ‘SUMO enigma’ [109
]. Although the underlying mechanisms for this effect on many SUMO-modified proteins have not been determined, elegant explanations based on the highly dynamic nature of the SUMOylation cycle have been proposed to account for this effect in some specific cases. For example, in neurons, the SUMO modification of GluR6 (glutamate receptor subunit 6) leads to its endocytosis at the plasma membrane [110
]. Although only a small proportion of SUMOylated GluR6 can be detected at a given time point, a large proportion of GluR6 undergoes SUMO-mediated endocytosis. The low level of detectable SUMOylated GluR6 is probably due to the action of SENPs rapidly removing SUMO. Nonetheless, the functional effects of GluR6 SUMOylation persist after deSUMOylation, namely GluR6 is endocytosed from the neuronal surface. Thus once SUMOylation has mediated endocytosis, SUMO can be removed and any GluR6 SUMOylated previously will have a different cellular localization to GluR6 that has never been modified.
Similar explanations have been used to account for the potent repression of transcription by protein SUMOylation. Multiple transcription factors can undergo SUMOylation and in many cases this leads to repression of transcription (for reviews see [111
]). For some transcription factors, such as Sp3 (stimulating protein 3) or LEF-1 (lymphoid enhancer-binding factor 1), SUMOylation causes their relocation to nuclear subdomains not associated with transcriptional activity [35
]. Thus, as with GluR6, the SUMOylation-mediated effects persist after the removal of SUMO.
SUMO modification of promoter-occupied transcription factors has also been reported to recruit chromatin-modifying proteins, such as the HDACs, or repressor proteins, such as DAXX (death-domain-associated protein) through non-covalent interactions [108
]. SUMO-mediated HDAC recruitment leads to local histone deacetylation, resulting in a more condensed chromatin structure that favours transcriptional repression. Thus recruitment of HDACs to a SUMOylated transcription factor will induce a repressive transcriptional environment that persists after removal of SUMO from the substrate. Similarly, recruitment of repressor proteins, such as DAXX, may lead to the formation of ‘repressor complexes’. Again, SUMO-mediated sequestration of transcription factors into repressive complexes would persist after removal of the SUMO.
The DNA-modifying enzyme TDG provides another example of how low-level substrate SUMOylation can result in robust effects. TDG recognizes and excises mismatched base pairs and SUMOylation is required for every enzymatic cycle [116
]. When TDG removes the mismatched base, it binds strongly to the DNA as a result of its high affinity for the abasic site created by the base excision [117
]. This high affinity interaction causes a break in the reaction cycle that is released by TDG SUMOylation, which decreases its affinity for DNA and releases it into the nucleoplasm [116
]. Subsequent removal of SUMO by SENPs allows TDG to undergo a further round of activity. Thus, although only a very small proportion of TDG is modified at any one time due to the rapid deconjugation of SUMO in the nucleoplasm, the effects of SUMOylation are critical to every reaction cycle.
Taken together these observations suggest that although only a small proportion of a substrate may be modified at any given time point, the majority of a substrate will be SUMOylated over time and undergo a SUMO-induced functional change that remains after the protein has been deSUMOylated.