van Esbroeck, A. C. M.; Verhagen, R. F. M.; Biagioni, M.; Fossati, M.; Distel, B.; Elgersma, Y.

UBE3A encodes three isoforms of Ubiquitin E3 ligase A, which differ in their N-terminal sequence, abundance, and localization. Recently, three individuals diagnosed with Angelman Syndrome have been described who carry a variant that abrogates the startcodon of the predominant nuclear isoform 1 (hUBE3A-Iso1p.Met1Thr) and concomitantly results in a missense variant in isoform 3 (hUBE3A-Iso1p.Met21Thr), which we previously reported to be nuclear enriched as well. Here, we studied the effect of the p.Met21Thr variant on hUBE3A-Iso3 localization. Recombinant expression of hUBE3A-Iso3p.Met21Thr in U2-OS and mouse neurons revealed similar UBE3A labelling in the nucleus and cytosol, indicating hUBE3A-Iso3 localization is sensitive to amino acid changes at this position. This finding prompted us to revisit hUBE3A-Iso3 localization, since we previously introduced a p.Met21Ala/p.Met22Ala amino acid substitution in hUBE3A-Iso3 and its mouse orthologue mUBE3A-Iso2 to prevent translation of the shorter hUBE3A-Iso1 and mUBE3A-Iso3 nuclear isoforms. Introduction of silent mutations to disfavour translation of the short UBE3A isoforms enabled us to determine the localization hUBE3A-Iso3 and mUBE3A-Iso2 in the absence of amino acid changes at the p.Met21/p.Met22 position respectively. Surprisingly, hUBE3A-Iso3 localization shifted from predominant nuclear localization for hUBE3A-Iso3p.Met21Ala to a predominant cytosolic localization of the Kozak optimized hUBE3A-Iso3KOZAK, while their mouse orthologues mUBE3A-Iso2p.Met22Ala and mUBE3A-Iso2KOZAK both localized predominantly to the cytosol. Taken together, these experiments indicate that the localization of human UBE3A-Iso3 is highly sensitive to amino acid substitutions at the p.Met21 position and that variants at this position not only abrogate the translation of hUBE3A-Iso1, but can also change the localization of hUBE3A-Iso3.