X-ray crystal structure of proto-oncogene product c-Rel bound to the CD28 response element of IL-2.
BACKGROUND
The proto-oncogene product c-Rel is a Rel/NF-kappaB family transcription factor that plays a critical role in lymphoid cell development and mediates CD28-induced expression of interleukin 2 (IL-2). The CD28 response element (CD28RE) in the IL-2 enhancer is nonameric and similar to the kappaB DNA target sites recognized by p65 homodimers.


RESULTS
We have determined and refined the X-ray crystal structure of the c-Rel homodimer complexed to the CD28RE DNA site, 5'-AGAAATTCC-3', to 2.85 A resolution. The c-Rel homodimer binds CD28RE in a mode similar to that observed in the p65/IL-8 kappaB crystallographic complex. Binding studies reveal that the c-Rel homodimer recognizes the CD28RE with higher affinity as compared to other canonical kappaB sequences despite the nonconsensus A:T base pair at the 5' end of the CD28RE. Preferential recognition of the CD28RE by c-Rel results from the direct contacts between the protein and the DNA as well as intrasubunit interactions between the beta(f)-beta(g) loop in the dimerization domain and the DNA-contacting loop L1 of the N-terminal domain. Not only do these loops have different conformations in other Rel/DNA crystallographic complexes, but they also contain two of the five oncogenic point mutations found in v-Rel.


CONCLUSIONS
The current structure indicates that a non-DNA-contacting loop in the dimerization domain and the DNA-contacting loop L1 may play critical roles in defining affinity and specificity. Two amino acid changes in these segments may account for the differential DNA binding by v-Rel as compared to that of c-Rel.
Huang
D B
DB
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093, USA.
Chen
Y Q
YQ
Ruetsche
M
M
Phelps
C B
CB
Ghosh
G
G
D000595:Amino Acid Sequence; D000818:Animals; D016880:Anisotropy; D001483:Base Sequence; D018106:CD28 Antigens / Q000737:chemistry*; D002645:Chickens; D018360:Crystallography, X-Ray; D004247:DNA / Q000378:metabolism; D019281:Dimerization; D004305:Dose-Response Relationship, Drug; D004926:Escherichia coli / Q000378:metabolism; D007376:Interleukin-2 / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D016328:NF-kappa B / Q000737:chemistry; D020572:Oncogene Proteins v-rel / Q000378:metabolism; D017354:Point Mutation; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D020569:Proto-Oncogene Proteins c-rel / Q000737:chemistry*; D017386:Sequence Homology, Amino Acid; D013050:Spectrometry, Fluorescence
D018106:CD28 Antigens; D007376:Interleukin-2; D016328:NF-kappa B; D020572:Oncogene Proteins v-rel; D020569:Proto-Oncogene Proteins c-rel; D004247:DNA