Molecular mechanisms of transcriptional control by Sp2

• Applicant: Professor Dr. Guntram Suske
• Subject Area: General Genetics and Functional Genome Biology ; Biochemistry ; Cell Biology
• Term: from 2011 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 200991012

A major question in eukaryotic gene regulation is how transcription factors with similar structural features elicit specific biological responses. We have used the three transcription factors Sp1, Sp2 and Sp3 as a paradigm for investigating this question. All three proteins are ubiquitously expressed, and they share glutamine-rich domains as well as a conserved zinc finger DNA binding domain. Yet, each of the three proteins carries out unique functions in vivo, and each is absolutely essential for mouse development. In the previous funding period, we have examined and compared the genomic binding patterns of Sp1, Sp2 and Sp3 in mouse embryonic fibroblasts and in a human cell line. A key finding was that Sp2 does not localize at GC boxes as do the related transcription factors Sp1 and Sp3, but instead localizes at tandemly arranged CCAAT motifs. A mechanistic clue for their different genomic binding site selection was obtained when we found that Sp1 and Sp3 on the one hand, and Sp2 on the other hand engage completely different protein domains for promoter recognition. Most strikingly, the zinc finger DNA binding domain of Sp2 is dispensable for recruitment to its target sites in vivo. We have identified the trimeric histone-fold CCAAT box binding transcription factor Nf-y as the major partner for Sp2-chromatin interaction. Nf-y is critical for recruitment of Sp2 to co-occupied regulatory elements. Equally, Sp2 potentiates binding of Nf-y to shared sites indicating the existence of an extensive Sp2-Nf-y interaction network. Altogether our results unveiled a striking and unexpected genomic loading mechanism of Sp2, and suggest that Sp2 acts as a cofactor rather than as a classical DNA-binding transcription factor. Analysis of published ChIP-seq data sets further revealed that the majority of the Sp2 target promoters are high occupancy target (HOT) regions. HOT regions are bound by many different transcription factors and cofactors. How transcription factors are recruited to HOT promoters that lack corresponding DNA binding motifs is not understood. Moreover, it is not known whether and how binding of factors to HOT regions impinges on each other. By following the mechanistic clues provided by our preliminary work on Sp2 we will address these important questions. Particularly, we aim to (i) delineate and characterize the domains mediating genomic binding of Sp2 to HOT promoters in vivo, (ii) analyze the DNA-sequence constraints that determine zinc finger independent genomic loading of Sp2 at HOT promoters (iii) identify and investigate occupancy of other factors at Sp2 HOT promoters, and (iv) analyze whether Sp2 affects recruitment of these factors, and vice versa. We believe that our work will have broader implications for the understanding of how bona fide DNA-binding transcription factors are recruited to HOT promoter regions that lack corresponding DNA binding motifs thereby acting as cofactors.

• DFG Programme: Research Grants