Fig. 1: Independent contribution of the SEC and DOT1L complexes to CSR.

a Protein–protein interaction network from BioGRID for AID-BirA-specific preys identified by BioID in Flp-In T-REx 293 cells. Nodes represent proteins; edges indicate known interactions. Selected complexes are highlighted. b Heat map of average spectral counts (n = 4 replicates) for selected chromatin-associated proteins detected in AID and APOBEC2 (A2) BioID experiments. W–D specificity scores for each prey are shown. SEC and DOT1L complex components are highlighted. c Composition of DOT1L, SEC, and SEC-L3 complexes. Arrows indicate known physical interactions. d Frequency of IgA⁺ CH12F3 cells expressing the indicated shRNAs, measured by flow cytometry 3 days after cytokine (CIT) stimulation. Mean values (bars) from n = 3 to 5 biological replicates (symbols). e Relative levels of unspliced Sμ and Sα germline transcripts (GLTs) in CH12F3 cells expressing the indicated shRNAs (red), normalized to control cells expressing a non-targeting shRNA (grey), measured by RT–qPCR 48 h post-CIT. Mean values (bars) from n = 3 biological replicates (symbols). Schematic of the Igh locus shows amplicon positions. f Western blot of the indicated proteins in total lysates from parental and DOT1L-deficient CH12F3 cell clones. Uncropped blots in Source Data. g Frequency of IgA⁺ cells in parental and DOT1L-deficient CH12F3 clones measured by flow cytometry 48 h post-CIT. Mean values (bars) from n = 4 biological replicates (symbols). h Frequency of IgA⁺ cells in WT or DOT1L-deficient CH12F3 cells expressing the indicated shRNAs, measured by flow cytometry 48 h post-CIT. Mean values (bars) from n = 3 biological replicates (symbols). i Mutation frequency in a region 5′ of the Sμ segment in CH12F3 cells. Pie charts show the proportion of sequences with the indicated number of mutations; total number of sequences is shown at center. d, g (lefthand side), h P values shown for significant differences by one-way ANOVA with Dunnett’s multiple comparison test; e, g (righthand side) by unpaired two-tailed Student t-test.