Competing protein-protein interactions regulate binding of Hsp27 to its client protein tau

Abstract

Small heat shock proteins (sHSPs) are a class of oligomeric molecular chaperones that limit protein aggregation. However, it is often not clear where sHSPs bind on their client proteins or how these protein-protein interactions (PPIs) are regulated. Here, we map the PPIs between human Hsp27 and the microtubule-associated protein tau (MAPT/tau). We find that Hsp27 selectively recognizes two aggregation-prone regions of tau, using the conserved β4-β8 cleft of its alpha-crystallin domain. The β4-β8 region is also the site of Hsp27–Hsp27 interactions, suggesting that competitive PPIs may be an important regulatory paradigm. Indeed, we find that each of the individual PPIs are relatively weak and that competition for shared sites seems to control both client binding and Hsp27 oligomerization. These findings highlight the importance of multiple, competitive PPIs in the function of Hsp27 and suggest that the β4-β8 groove acts as a tunable sensor for clients.

Designing Molecular Spanners to Throw in the Protein Networks

“…Recently, Freilich et al. have studied the different interactions of a representative of this class, namely Hsp27, with the microtubule‐associated protein tau . Misfolding and aggregation of tau are at the basis of several neuropathies.…”

“…Misfolding and aggregation of tau are at the basis of several neuropathies. Interestingly, Hsp27 was shown to interact with two different aggregation‐prone regions of tau through a conserved cleft, which incidentally also mediates self‐interaction (Hsp27–Hsp27) …”

Dysregulated interactions triggered by a neuropathy-causing mutation in the IPV motif of HSP27

“…Thus, were the IxI/V binding affinity to be lowered, subunit exchange would occur more slowly and the average oligomer size would increase (24,25). Previous studies independently support such a mechanism, as other HSP27 mutants in the IxI/V motif have been shown to increase the molecular mass, including the HSP27 variants P182S (40) and GPG (34), and other IxI/V mutations (32)(33)(34)(35). In addition, it has been demonstrated that other regions of sHSPs can also interact with the ACD, including NTD-β4/β8 groove interactions observed in a crystal structure of HSPB6 (69) and NTD-ACD dimer interface contacts as seen in a crystal structure of the HSPB2-HSPB3 hetero-tetramer (70).…”

“…Previous solution-state NMR studies of WT HSP27 only observed resonances from the disordered CTR, and thus could not probe CTR binding to the ACD in context of the fulllength protein (47,48). We therefore turned to the isolated ACD, which forms stable dimers (2 x 10 kDa) that are known to bind to a peptide encompassing the IxI/V motif of the CTR (20,22,31,33,34,37).…”

“…Despite mediating the ACD-CTR interaction, the IxI/V motif is not necessarily a prerequisite for oligomeric assembly in HSP27 or the related sHSPs αA-and αB-crystallin. Indeed, mutations that attenuate or prevent IxI/V binding did not decrease the average oligomeric size (30)(31)(32)(33)(34)(35). In addition, the IxI/V motif can facilitate the interaction of HSP27 with other proteins that contain this motif, including the HSP70 cochaperone Bcl-2-associated anthanogene-3 (BAG3), which plays a significant role in regulating apoptosis and autophagy (36).…”