Dimerization region structure

Figure 10.25 Structure of the dimerization region of MyoD. The a helices HI (red and brown) and H2 (light and dark green) of the two monomers form a four-helix bundle that keeps the dimer together. The loops (yellow and orange) are on the outside of the four-helix bundle. (Adapted from P.C.M. Ma et al.. Cell 77 4S1-4S9, 1994.)...

Helix-loop-helix (b/HLH) transcription factors are either heterodimers or homodimers with basic a-helical DNA-binding regions that lie across the major groove, rather than along it, and these helices extend into the four-helix bundle that forms the dimerization region. A modification of the b/HLH structure is seen in some transcription factors (b/HLH/zip) in which the four-helix bundle extends into a classic leucine zipper. 

Figure 14.27 EGF receptor dimerization. The structure of the extracellular region of the EGF receptor is shown bound to EGF. Notice that the structure is dimeric with one EGF molecule bound to each receptor molecule and that the dimerization is mediated by a dimerization arm that extends from each receptor molecule.

Calculations of the spectra of the closely related compounds—acetylsalicylic acid (aspirin) and salicylaldehyde, at different temperatures and with selective deuteration, have been published for acetylsalicylic acid in Ref. 54 and for salicylaldehyde in Ref. 53. In the acetylsalicylic acid crystal there are no intramolecular hydrogen bonds—only intermolecular hydrogen bonds are present. The molecules of acetylsalicylic acid form hydrogen-bonded dimeric cyclic structures, similar to those present in salicylic acid. The IR spectra of the acetylsalicylic acid and the salicylic acid crystals in the region of the O—H(D) stretching vibrations are similar, excluding peaks due to intramolecular hydrogen bonds. Spectra of the acetylsalicylic acid crystal were calculated at two temperatures 300 and 77K. The experimental spectra and the observed temperature and isotopic effects are reproduced in these calculations reasonably well [54]. 

Figure 8.11 The DNA-binding domain of 434 repressor. It is a dimer in its complexes with DNA fragments. Each subunit (green and brown) folds into a bundle of four a helices (1-4) that have a structure similar to the corresponding region of the lambda repressor (see Figure 8.7) including the helix-turn-helix motif (blue and red). A fifth a helix (5) is involved in the subunit interactions, details of which are different from those of the lambda repressor fragment. The structure of the 434 Cro dimer is very similar to the 434 repressor shown here.

Subsequently Stephen Harrison s group determined the x-ray structure of a PPRl-DNA complex and showed that the zinc cluster domain of PPRl and its mode of binding to DNA was very similar to that of GAL4, and that PPRl also dimerized through a coiled-coil region. However, the linker region... 

The coiled-coil structure of the leucine zipper motif is not the only way that homodimers and heterodimers of transcription factors are formed. As we saw in Chapter 3 when discussing the RNA-binding protein ROP, the formation of a four-helix bundle structure is also a way to achieve dimerization, and the helix-loop-helix (HLH) family of transcription factors dimerize in this manner. In these proteins, the helix-loop-helix region is preceded by a sequence of basic amino acids that provide the DNA-binding site (Figure 10.23), and... 

---