SUMO-1 modification of PIASy, an E3 ligase, is necessary for PIASy-dependent activation of Tcf-4

We have previously shown that modification of Tcf-4, a transcription factor in the Wnt pathway, with SUMO by PIASy, a SUMO E3 ligase, enhances its transcriptional activity. Since PIASy itself was also modified with SUMO-1, we studied the role of sumoylation of PIASy in the regulation of Tcf-4. Lys ³⁵ was found to be a sumoylation site of PIASy. PIASy^K35R, in which Lys³⁵ was mutated to Arg, did not enhance sumoylation of Tcf-4, although this PIASy mutant did not lose the ligase activity of sumoylation for other proteins. Wild-type PIASy and PIASy^K35R showed a distinct distribution in the nucleus, although both were colocalized with Tcf-4. Promyelocytic leukemia protein, which is involved in transcriptional regulation, was associated with PIASy^K35R more frequently than wild-type PIASy in the nucleus. PIASy^K35R could not stimulate the transcriptional activity of Tcf-4 under the conditions in which wild-type PIASy enhanced it. Conjugation of SUMO-1 to the amino terminus of PIASy^K35R neither enhanced sumoylation of Tcf-4 nor stimulated the transcriptional activity of Tcf-4. These results suggest that sumoylation of Lys³⁵ in PIASy determines the nuclear localization of PIASy and that it is necessary for PIASy-dependent sumoylation and transcriptional activation of Tcf-4.