Fig 1: SEI1 bridges the interaction between CBP/p300 and PAF1 complex to regulate PD‐L1 transcription. A) HA‐SEI1 overexpressed IM‐9 cells were lysed, and the total protein lysate was immunoprecipitated with an agarose‐immobilized HA antibody and analysis with mass spectrometry. The proteins identified are indicated. B,C) Co‐immunoprecipitation of SEI1, PAF1, CDC73, or p300 in ARH‐77 or IM‐9 cells treated with melphalan or bortezomib. D) Co‐immunoprecipitation of SEI1 along with PAF1, CDC73, or p300 in HEK293T cells co‐transfected with SEI1 and either PAF1, CDC73, or p300 plasmid. E) Schematic of the truncations including ΔCABD, ΔSD, ΔPBID, and ΔCT fragments. F) Western blotting showing different truncations of SEI1 (full length, ΔCABD, ΔSD, ΔPBID, and ΔCT) in HEK293T cells. G–K) Pulldown of PAF1, CDC73, or p300 with different truncations of SEI1 (full length, ΔCABD, ΔSD, ΔPBID, and ΔCT) in HEK293T cells. L) Pulldown of MYC‐p300 with CDC73 or PAF1 in HEK293T cells. M) Gene tracks showing representative ATAC‐seq profiles at PD‐L1 gene loci in primary CD138+ plasma cells from two relapsed myeloma patients following treatment with bortezomib. N,O) ChIP PCR assay showing SEI1, p300, or H3K27ac enrichment on PD‐L1 promoter of ARH‐77 or IM‐9 cells treated with or without melphalan or bortezomib. P) ChIP‐seq profiles show H3K27ac signal tracks at the PD‐L1 gene loci for ARH‐77 cells treated with or without melphalan or bortezomib. Q) ChIP PCR assay showing H3K27ac enrichment on PD‐L1 promoter of ARH‐77 or IM‐9 cells treated with or without melphalan or bortezomib. R) ChIP PCR assay showing enrichment of H3K27ac on PD‐L1 promoter in shCtrl or shSEI1 ARH‐77 or IM‐9 cells. S) Expression of PAF1 in nonspecific, PAF1 shRNA‐expressing myeloma cells. T,U) The relative expression of PD‐L1 in PAF1 knockdown ARH‐77 and IM‐9 cells treated with or without melphalan or bortezomib. V,W) ChIP PCR assay showing enrichment of V) SEI1 or W) H3K27ac on PD‐L1 promoter in shCtrl or shPAF1 ARH‐77 and IM‐9 cells. X) Schematic of the SEI1 bridges the interaction between CBP/p300 and PAF1 complex to regulate PD‐L1 transcription. B–D,F–L,N,O,Q–W) Representative of three independent experiments. Data are averages ± SD. ***p < 0.001 and ****p < 0.0001. All p‐values were determined using one way ANOVA.