The effect of the acidic tail on the DNA-binding properties of the HMG1,2 class of proteins: Insights from tail switching and tail removal

The high-mobility group (HMG) proteins HMG1, HMG2 and HMG2a are relatively abundant vertebrate DNA-binding and bending proteins that bind with structu re specificity, rather than sequence specificity, and appear to play an arc hitectural role in the assembly of nucleoprotein complexes. They have two h omologous "HMG-box" DNA-binding domains (which show about 80 % homology) co nnected by a short basic linker to an acidic carboxy-terminal tail that dif fers in length between HMG1 and 2. To gain insights into the role of the ac idic tail, we examined the DNA-binding properties of HMG1, HMG2b and HMG2a from chicken erythrocytes (corresponding to HMG1, HMG2 and HMG2a in other v ertebrates). HMG1, with the longest acidic tail, is less effective than HMG 2a and 2b (ata given molar input ratio) in supercoiling relaxed, closed cir cular DNA, in inducing ligase-mediated circularisation of an 88 bp DNA frag ment, and in binding to four-way DNA junctions in a gel-shift assay. Remova l of the acidic tail increases the affinity of the HMG boxes for DNA and la rgely abolishes the differences between the three species. Switching the ac idic tail of HMG1 for that of HMG2a or 2b gives hybrid proteins with essent ially the same DNA-binding properties as HMG2a, 2b. The length (and possibl y sequence) of the acidic tail thus appears to be the dominant factor in me diating the differences in properties between HMG1, 2a and 2b and finely tu nes the rather similar DNA-binding properties of the tandem HMG boxes, pres umably to fulfill different cellular roles. The tail is essential for struc ture-selective DNA-binding of the HMG boxes to DNA minicircles in the prese nce of equimolar linear DNA, and has little effect on the affinity for this already highly distorted DNA ligand, in contrast to binding to linear and four-way junction DNA. (C) 2000 Academic Press.