Transcriptional Scaffold: CIITA Interacts with NF-Y, RFX, and CREB To Cause Stereospecific Regulation of the Class II Major Histocompatibility Complex Promoter

Scaffold molecules interact with multiple effectors to elicit specific signal transduction pathways. CIITA, a non-DNA-binding regulator of class II major histocompatibility complex (MHC) gene transcription, may serve as a transcriptional scaffold. Regulation of the class II MHC promoter by CIITA requires strict spatial-helical arrangements of the X and Y promoter elements. The X element binds RFX (RFX5/RFXANK-RFXB/RFXAP) and CREB, while Y binds NF-Y/CBF (NF-YA, NF-YB, and NF-YC). CIITA interacts with all three. In vivo analysis using both N-terminal and C-terminal deletion constructs identified critical domains of CIITA that are required for interaction with NF-YB, NF-YC, RFX5, RFXANK/RFXB, and CREB. We propose that binding of NF-Y/CBF, RFX, and CREB by CIITA results in a macromolecular complex which allows transcription factors to interact with the class II MHC promoter in a spatially and helically constrained fashion. The major histocompatibility complex (MHC) class II proteins play a central role in the immune response. Extensive analysis has underscored that much of the fluctuation in class II MHC antigen expression can be attributed to changes at the transcriptional level (46, 47). In addition to the class II MHC molecules themselves, associative accessory molecules that are necessary for class II antigen MHC function appear to be controlled in a similar fashion. These associative molecules include the MHC class II-associated invariant chain (Ii) and the more recently described DM heterodimer. All class II MHC, Ii, and DM promoters share the unique presence of three DNA elements, called W, X, and Y, which are highly conserved and critical for promoter function (2, 15). The WX-Y elements are not only important for constitutive gene expression in B cells but also critical for inducible gene expression. In addition to the conservation in sequence, the spacing between the X and Y elements is highly conserved at approximately two helical turns. Increasing the number of helical turns between these two elements preserves function, while disrupting this orientation destroys promoter activation. Our group previously hypothesized that this restrictive spacing may be required to align the X and Y elements on the same side of the DNA helix, thus allowing transcription factors which can bind these elements to directly interact or to participate in the assembly of a larger promoter complex (48, 49). The Y box is a CCAAT motif, and it interacts with NF-Y/ CBF (also known as YEBP/CP-1). NF-Y/CBF is composed of A, B, and C subunits (26, 27, 57), with the conserved core sequences of NF-YC (CBF-C) and NF-YB (CBF-A) forming a