Transcription factor binding at Ig enhancers is linked to somatic hypermutation targeting.

Secondary diversification of the Ig repertoire occurs through somatic hypermutation (SHM), gene conversion (GCV), and class switch recombination (CSR)-three processes that are initiated by activation-induced cytidine deaminase (AID). AID targets Ig genes at orders of magnitude higher than the rest of the genome, but the basis for this specificity is poorly understood. We have previously demonstrated that enhancers and enhancer-like sequences from Ig genes are capable of stimulating SHM of neighboring genes in a capacity distinct from their roles in increasing transcription. Here, we use an in vitro proteomics approach to identify E-box, MEF2, Ets, and Ikaros transcription factor family members as potential binders of these enhancers. ChIP assays in the hypermutating Ramos B cell line confirmed that many of these factors bound the endogenous Igλ enhancer and/or the IgH intronic enhancer (Eµ) in vivo. Further investigation using SHM reporter assays identified binding sites for E2A and MEF2B in Eµ and demonstrated an association between loss of factor binding and decreases in the SHM stimulating activity of Eµ mutants. Our results provide novel insights into trans-acting factors that dictate SHM targeting and link their activity to specific DNA binding sites within Ig enhancers.

The Arabidopsis O-fucosyltransferase SPINDLY activates nuclear growth repressor DELLA.

Plant development requires coordination among complex signaling networks to enhance the plant's adaptation to changing environments. DELLAs, transcription regulators originally identified as repressors of phytohormone gibberellin signaling, play a central role in integrating multiple signaling activities via direct protein interactions with key transcription factors. Here, we found that DELLA is mono-O-fucosylated by the novel O-fucosyltransferase SPINDLY (SPY) in Arabidopsis thaliana. O-fucosylation activates DELLA by promoting its interaction with key regulators in brassinosteroid- and light-signaling pathways, including BRASSINAZOLE-RESISTANT1 (BZR1), PHYTOCHROME-INTERACTING-FACTOR3 (PIF3) and PIF4. Moreover, spy mutants displayed elevated responses to gibberellin and brassinosteroid, and increased expression of common target genes of DELLAs, BZR1 and PIFs. Our study revealed that SPY-dependent protein O-fucosylation plays a key role in regulating plant development. This finding may have broader importance because SPY orthologs are conserved in prokaryotes and eukaryotes, thus suggesting that intracellular O-fucosylation may regulate a wide range of biological processes in diverse organisms.

O-GlcNAcylation of master growth repressor DELLA by SECRET AGENT modulates multiple signaling pathways in Arabidopsis.

The DELLA family of transcription regulators functions as master growth repressors in plants by inhibiting phytohormone gibberellin (GA) signaling in response to developmental and environmental cues. DELLAs also play a central role in mediating cross-talk between GA and other signaling pathways via antagonistic direct interactions with key transcription factors. However, how these crucial protein-protein interactions can be dynamically regulated during plant development remains unclear. Here, we show that DELLAs are modified by the O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) SECRET AGENT (SEC) in Arabidopsis. O-GlcNAcylation of the DELLA protein REPRESSOR OF ga1-3 (RGA) inhibits RGA binding to four of its interactors-PHYTOCHROME-INTERACTING FACTOR3 (PIF3), PIF4, JASMONATE-ZIM DOMAIN1, and BRASSINAZOLE-RESISTANT1 (BZR1)-that are key regulators in light, jasmonate, and brassinosteroid signaling pathways, respectively. Consistent with this, the sec-null mutant displayed reduced responses to GA and brassinosteroid and showed decreased expression of several common target genes of DELLAs, BZR1, and PIFs. Our results reveal a direct role of OGT in repressing DELLA activity and indicate that O-GlcNAcylation of DELLAs provides a fine-tuning mechanism in coordinating multiple signaling activities during plant development.