• α-crystallin;
  • α-heat shock protein;
  • small heat shock protein;
  • chaperone;
  • oligomerization

Oligomerization into multimeric complexes is a prerequisite for the chaperone function of almost all α-crystallin type heat shock proteins (α-Hsp), but the molecular details of complex assembly are poorly understood. The α-Hsp proteins from Bradyrhizobium japonicum are suitable bacterial models for structure-function studies of these ubiquitous stress proteins. They fall into two distinct classes, A and B, display chaperone activity in vitro and form oligomers of ≈ 24 subunits. We constructed 19 derivatives containing truncations or point mutations within the N- and C-terminal regions and analyzed them by gel filtration, citrate synthase assay and coaffinity purification. Truncation of more than the initial few amino acids of the N-terminal region led to the formation of distinct dimeric to octameric structures devoid of chaperone activity. In the C-terminal extension, integrity of an isoleucine-X-isoleucine (I-X-I) motif was imperative for α-Hsp functionality. This I-X-I motif is one of the characteristic consensus motifs of the α-Hsp family, and here we provide experimental evidence of its structural and functional importance. α-Hsp proteins lacking the C-terminal extension were inactive, but still able to form dimers. Here, we demonstrate that the central α-crystallin domain alone is not sufficient for dimerization. Additional residues at the end of the N-terminal region were required for the assembly of two subunits.