Basal transcription of many genes in yeast is

A key event in the response of cells to proliferative signals is the rapid, transient induction of the c-fos proto-oncogene, which is mediated through the serum response element (SRE) in the fos promoter. Genomic footprinting and transfection experiments suggest that this activation occurs through a ternary complex that includes the serum response factor (SRF) and the ternary complex factor p62. Interaction of p62TCF with the SRF-SRE binary complex requires a CAGGA tract immediately upstream of the SRE. Proteins of the ets proto-oncogene family bind to similar sequences and we have found that a member of this family, Elk-1, forms SRF-dependent ternary complexes with the SRE. Elk-1 and p62TCF have the same DNA sequence requirements and antibodies against Elk-1 block the binding of both proteins. Furthermore, we show that like p62TCF, Elk-1 forms complexes with the yeast SRF-homologue MCM1 but not with yeast ARG80. But ARG80 mutants that convey interaction with p62TCF can also form complexes with Elk-1. The similarity, or even identity, between Elk-1 and p62TCF suggests a novel regulatory role for Ets proteins that is effected through interaction with other proteins, such as SRF. Furthermore, the possible involvement of an Ets protein in the control of c-fos has interesting implications for proto-oncogene cooperation in cellular growth control.

Lymph nodes (LNs) are strategically situated throughout the body at junctures of the blood vascular and lymphatic systems to direct immune responses against antigens draining from peripheral tissues. The current paradigm describes LN development as a programmed process that is governed through the interaction between mesenchymal lymphoid tissue organizer (LTo) cells and hematopoietic lymphoid tissue inducer (LTi) cells. Using cell-type-specific ablation of key molecules involved in lymphoid organogenesis, we found that initiation of LN development is dependent on LTi-cell-mediated activation of lymphatic endothelial cells (LECs) and that engagement of mesenchymal stromal cells is a succeeding event. LEC activation was mediated mainly by signaling through receptor activator of NF-κB (RANK) and the non-canonical NF-κB pathway and was steered by sphingosine-1-phosphate-receptor-dependent retention of LTi cells in the LN anlage. Finally, the finding that pharmacologically enforced interaction between LTi cells and LECs promotes ectopic LN formation underscores the central LTo function of LECs.

MCM1 and SRF bind to the same DNA sequence and form ternary complexes with STE12 and p62TCF, respectively. We show that in gel retardation assays, MCM1 recruits both ternary complex factors whereas SRF interacts only with p62TCF. A protein domain of 90 amino acids, shared by MCM1 and SRF, was found to be sufficient for ternary complex formation. The domain is also required for dimerization and DNA binding. Similar regions are found in other proteins, such as ARG80, Deficiens and Agamous. ARG80 and Agamous exhibit similar DNA binding specificities but do not interact with either STE12 or p62TCF. By exchanging three residues of ARG80 with those of corresponding positions in SRF (residues 198, 200 and 203), the ARG80 protein acquires the ability to recruit p62TCF into a ternary complex. Likewise, the substitution of four SRF amino acids by MCM1‐derived residues (amino acids 73, 75, 77 and 78) confers on SRF the ability to interact with STE12. Thus, we have identified specific amino acids in MCM1 and SRF that are critical for ternary complex formation and which map to equivalent positions within the shared domains. Therefore, the structural basis for specific protein‐protein interaction appears to be conserved in evolution between a class of transcription factors.