Also, the ability of HSPB1 to bind to a wide spectrum of substrates has been extensively described 50, 64, 65, 66, 67, avoiding the formation and elongation of amyloid fibrils, preventing aggregation of misfolded polypeptides and indirectly promoting their refolding or degradation. HSPB1 is ubiquitously expressed in different tissues, especially under stress conditions 62, 63, 64. Nevertheless, human HSPB1 may still be an obvious candidate. Human HSPB5 was reported to promote disaggregation of α-synuclein by HSC70 (the constitutive form of HSP70) and co-chaperones, but was not involved in the co-aggregation step, since it was added to pre-formed fibers furthermore, it resulted in depolymerization over 10 days 61, much slower than the disaggregation in 1–3 h typically observed with HSC70 or HSP70 24, 26, 27, 28.

However, in the absence of co-chaperones it is unclear whether actual disaggregation was being observed, as opposed to refolding of soluble substrate that had a high rate of spontaneous dissociation from complexes. Mouse Hspb1 formed large oligomeric complexes with denatured citrate synthase, which could then be reactivated by Hsp70 60. However, this has not yet been demonstrated.ĭue to the low sequence homology between sHSP across species, it is difficult to predict which human sHSP homolog of Hsp26 or IbpA/IbpB might be involved in disaggregation. The HSP100 disaggregases are absent in metazoans, so it is possible that sHSPs have a conserved role in co-aggregating with substrates to promote efficient disaggregation by the metazoan HSP70 system alone. Yeast and bacterial sHSPs, Hsp26 24, 26 and IbpA/IbpB 36, 37, 38 respectively, have been reported to co-aggregate with substrates, changing aggregate size, shape and composition, and increasing the efficiency of disaggregation by the combined HSP70 and HSP100 machinery 20, 22, 23, 26, 38, 39, 40, 41. In a separate function, sHSPs can be trapped inside protein aggregates, to facilitate downstream disaggregation. Members of the small heat shock protein (sHSP) family are ATP-independent chaperones, present in all organisms from bacteria to humans, and able to bind to early-unfolding intermediates of substrates, preventing their uncontrolled aggregation 33, 34, 35.

Also, these experiments suggest that the nucleotide exchange factors (NEFs) HSP110 or its close homolog APG2 are able to support HSP70 disaggregation activity by promoting ADP release and re-binding of ATP, which causes substrate release 24, 26, 28, 29, 30.Īlthough, traditionally, protein aggregation has been seen as an uncontrolled process in cells, many cellular factors have been identified as responsible for modulating the aggregation process 11, 31, 32. The J-domain proteins DJA2 and DJB1 are thought to cooperate to bind the aggregates, recruit HSP70 and stimulate its ATP hydrolysis and substrate polypeptide binding 24, 26, 28, 29. Previous work has shown that in humans and other metazoans, the chaperone HSP70 together with its co-chaperones DNAJB1 (DJB1), DNAJA2 (DJA2) and HSP110/APG2 are capable of disaggregation 24, 25, 26, 27, 28. Disaggregation performed mainly by molecular chaperones, likely the fastest process, is generally followed by protein refolding or proteasomal degradation 12, 21, 22, 23. In order to cope with these aggregates, cells have developed different strategies 11, 12, including compartmentalization into aggresomes followed by autophagy 13, 14, 15, secretion to the extracellular environment 16 and disaggregation (or resolubilization) 17, 18, 19, 20. Therefore, we propose that a human sHSP is an integral part of the chaperone network for protein disaggregation. HSPB1 itself was also extracted during disaggregation, and its homo-oligomerization ability was not required. Importantly, co-aggregation promoted the efficient disaggregation and refolding of the substrates, led by HSP70. The co-aggregates formed with HSPB1 were smaller and more regularly shaped than those formed in its absence. HSPB1 co-aggregated with unfolded protein substrates, firefly luciferase and mammalian lactate dehydrogenase. Because it is unknown whether a human sHSP has this activity, we investigated the disaggregation role of human HSPB1. The co-aggregated sHSPs then facilitate downstream disaggregation by HSP70. Yeast and bacterial chaperones of the small heat-shock protein (sHSP) family can bind substrates at early stages of misfolding, during the aggregation process. To eliminate aggregates, the HSP70 chaperone machinery extracts and resolubilizes polypeptides for triage to refolding or degradation. In human cells under stress conditions, misfolded polypeptides can form potentially cytotoxic insoluble aggregates.

3d comic the chaperone. episode 16