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

Rebecca Freilich,Olivier Julien,Sue-Ann Mok,Eric Tse,Miguel Betegon,David A Agard,Daniel R Southworth,Jason E Gestwicki,Koh Takeuchi

doi:10.1038/s41467-018-07012-4

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.

Highlights

Small heat shock proteins are a class of oligomeric molecular chaperones that limit protein aggregation
We utilized two isoforms of tau (Fig. 1a and Supplementary Figure 1): the full-length splice variant 0N4R tau, which is commonly expressed in adult brain and K18, a construct containing just the four microtubulebinding repeats (MTBRs) that is known to contain the binding sites of other chaperones[34,35]
We found that both tau isoforms caused chemical shift perturbations (CSPs; >0.4 ppm) within the hydrophobic β4–β8 groove of the a highly conserved α-crystallin domain (ACD), in addition to sporadic shifts along the dimer interface (Fig. 1b)

Summary

Introduction

Small heat shock proteins (sHSPs) are a class of oligomeric molecular chaperones that limit protein aggregation. The involvement of the β4–β8 groove was unexpected and interesting because, as mentioned above, this region is known to bind IXI motifs within the sHSP oligomer This result suggests that competition between client and IXI might limit chaperone function. We find that ACD binding alone is not sufficient to prevent tau aggregation and that additional contacts in the NTD are required Together, these results suggest that, at least for tau, Hsp27’s NTD plays a major role in chaperone function, while the β4–β8 groove serves as an important client sensor. These results suggest that, at least for tau, Hsp27’s NTD plays a major role in chaperone function, while the β4–β8 groove serves as an important client sensor This model advances our understanding of sHSP structure-function and suggests potential ways of targeting Hsp[27] for drug discovery

Methods

Results

Conclusion