Fig 1: Identification of XPO4 as a potential nuclear importer of phosphorylated UBE2A/B. (A) Interaction of XPO4 with phospho-mimetic UBE2A-SBP. FLAG-tagged RAD18 (positive control), KPNB1, TNPO1, TNPO2, and XPO4, together with SBP-tagged UBE2A-SBP S120D, were expressed in HEK293A cells for 24 h. Cell lysates were incubated with NHS-streptavidin beads for pull-down. Bound FLAG-tagged transport proteins were detected by Western blot using an anti-FLAG antibody, and UBE2A-SBP was detected using an anti-UBE2A/B antibody. (B) XPO4 preferentially interacts with phospho-mimetic UBE2A-SBP. FLAG-tagged XPO4 and SBP-tagged wild-type or S120D UBE2A-SBP were co-expressed in HEK293A cells. Cell lysates were subjected to SBP pull-down using NHS-streptavidin beads. Bound FLAG-XPO4 was detected by Western blot using an anti-FLAG antibody. Quantification shows a significant increase in interaction with the S120D mutant compared with WT and control (N = 3; one-way ANOVA, * p < 0.05, ** p < 0.01). (C) Phosphatase inhibition enhances the interaction between XPO4 and UBE2A. FLAG-tagged XPO4 and SBP-tagged wild-type or S120A UBE2A were expressed in HEK293A cells. After 24 h, cells were treated with phosphatase inhibitor cocktail C (Santa Cruz, sc-45065) or DMSO for 30 min. For the phosphatase inhibitor treatment group, an additional phosphatase inhibitor cocktail (MCE, HY-K0021) was added to the TTBS lysis buffer during cell lysis and incubation. Bound FLAG-XPO4 was detected by Western blotting using an anti-FLAG antibody (N = 3; two-way ANOVA, Tukey’s multiple comparisons test, * p < 0.05, ns: no significant difference.). (D,E) Predicted structure of the UBE2A-XPO4 complex generated using AlphaFold 3. Cyan, UBE2A; salmon, XPO4. The interface residues are opaque, while the rest are transparent. (F,G) Magnified views showing the side-chain interactions between the N-terminal basic helix of UBE2A and XPO4. Panel D shows the interaction between UBE2A Arg7 and XPO4 Gln979. Panel E shows the interaction between UBE2A Lys14 and XPO4 Ile990. (H,I) Structural comparison of phosphorylated and non-phosphorylated UBE2A in complex with XPO4. A potential interaction involving UBE2A pSer120 and Asp90 with XPO4 Lys260 is observed specifically in the phosphorylated UBE2A-XPO4 complex but not in the non-phosphorylated complex.
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