Allosteric Coupling via Communication of Distal Disorder-To-Order Transitions

Abstract

Disorder-to-order transitions in proteins can play critical roles in regulating function. Corepressor bio-5'-AMP binding to the E. coli biotin repressor promotes homodimerization that is a prerequisite to sequence-specific DNA binding and resulting transcription repression. Folding of a loop segment comprised of residues 211-222 in the corepressor binding site is required to obtain the full −4.0 kcal/mole energetic coupling between bio-5'-AMP binding and dimerization. Several additional loops, some of which are disordered in the unliganded monomer but folded in the liganded dimer, participate directly in the homodimer interface. Alanine scanning mutagenesis has demonstrated the functional significance of these loops for dimerization. Replacement of a glycine at position 142 in loop 140-146 with alanine results in complete abolition of coupling of corepressor binding to dimerization. Consistent with solution measurements and in contrast to wild type BirA, the structure of this variant solved by x-ray crystallography reveals a monomeric liganded protein. Moreover, the alanine replacement results in adoption of an alternative conformation by the 140-146 loop. Furthermore, distinct from wild type corepressor-bound BirA, both a neighboring dimer interface loop comprised of residues 193-199 and the 211-222 loop that is located 33 Å away from the alanine replacement are disordered in the liganded variant. These results support a model for energetic coupling between ligand binding and dimerization in which distant disorder-to-order transitions are communicated through the folded protein core.