Specific Sequences in the N-terminal Domain of Human Small Heat Shock Protein HSPB6 Dictate Preferential Heterooligomerization with the Orthologue HSPB1 [Protein Structure and Folding]

Small heat shock proteins (sHSPs) are a conserved group of molecular chaperones with important roles in cellular proteostasis. Although sHSPs are characterized by their small monomeric weight, they typically assemble into large polydisperse oligomers that vary in both size and shape but are principally composed of dimeric building blocks. These assemblies can comprise different sHSP orthologues, creating additional complexity that may affect chaperone activity. However, the structural and functional properties of such heterooligomers are poorly understood. We became interested in heterooligomer formation between human heat shock protein family B (small) member 1 (HSPB1) and HSPB6, which are both highly expressed in skeletal muscle. When mixed in vitro, these two sHSPs form a polydisperse oligomer array composed solely of heterodimers, suggesting preferential association that is determined at the monomer level. Previously, we have shown that the sHSP N-terminal domains (NTDs), which have a high degree of intrinsic disorder, are essential for the biased formation. Here we employed iterative deletion mapping to elucidate how the NTD of HSPB6 influences its preferential association with HSPB1 and show that this region has multiple roles in this process. First, the highly conserved motif RLFDQxFG is necessary for subunit exchange among oligomers. Second, a site approximately 20 residues downstream of this motif determines the size of the resultant heterooligomers. Third, a region unique to HSPB6 dictates the preferential formation of heterodimers. In conclusion, the disordered NTD of HSPB6 helps regulate the size and stability of heterooligomeric complexes, indicating that terminal sHSP regions define the assembly properties of these proteins.

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