Regulation of stress-induced intracellular sorting and chaperone function of Hsp27 (HspB1) in mammalian cells.
In vitro, small Hsps (heat-shock proteins) have been shown to have chaperone function capable of keeping unfolded proteins in a form competent for Hsp70-dependent refolding. However, this has never been confirmed in living mammalian cells. In the present study, we show that Hsp27 (HspB1) translocates into the nucleus upon heat shock, where it forms granules that co-localize with IGCs (interchromatin granule clusters). Although heat-induced changes in the oligomerization status of Hsp27 correlate with its phosphorylation and nuclear translocation, Hsp27 phosphorylation alone is not sufficient for effective nuclear translocation of HspB1. Using firefly luciferase as a heat-sensitive reporter protein, we demonstrate that HspB1 expression in HspB1-deficient fibroblasts enhances protein refolding after heat shock. The positive effect of HspB1 on refolding is completely diminished by overexpression of Bag-1 (Bcl-2-associated athanogene), the negative regulator of Hsp70, consistent with the idea of HspB1 being the substrate holder for Hsp70. Although HspB1 and luciferase both accumulate in nuclear granules after heat shock, our results suggest that this is not related to the refolding activity of HspB1. Rather, granular accumulation may reflect a situation of failed refolding where the substrate is stored for subsequent degradation. Consistently, we found 20S proteasomes concentrated in nuclear granules of HspB1 after heat shock. We conclude that HspB1 contributes to an increased chaperone capacity of cells by binding unfolded proteins that are hereby kept competent for refolding by Hsp70 or that are sorted to nuclear granules if such refolding fails.
Bryantsev
Anton L
AL
Department of Electron Microscopy, Belozersky Institute of Physico-Chemical Biology, Moscow State University, Moscow, 119992, Russia.
Kurchashova
Svetlana Yu
SY
Golyshev
Sergey A
SA
Polyakov
Vladimir Yu
VY
Wunderink
Herman F
HF
Kanon
Bart
B
Budagova
Karina R
KR
Kabakov
Alexander E
AE
Kampinga
Harm H
HH
D000818:Animals; D002460:Cell Line; D022003:Cell Nucleus Structures / Q000737:chemistry / Q000378:metabolism; D055551:HSP27 Heat-Shock Proteins; D018840:HSP70 Heat-Shock Proteins / Q000502:physiology; D006360:Heat-Shock Proteins / Q000502:physiology*; D018869:Heat-Shock Response / Q000502:physiology*; D007424:Intracellular Fluid / Q000737:chemistry / Q000502:physiology*; D051379:Mice; D018832:Molecular Chaperones / Q000502:physiology*; D009363:Neoplasm Proteins / Q000502:physiology*; D018384:Oxidative Stress / Q000502:physiology; D017510:Protein Folding; D021382:Protein Sorting Signals / Q000502:physiology; D021381:Protein Transport / Q000502:physiology; D051381:Rats
D055551:HSP27 Heat-Shock Proteins; D018840:HSP70 Heat-Shock Proteins; C057725:HSPB1 protein, human; D006360:Heat-Shock Proteins; D018832:Molecular Chaperones; D009363:Neoplasm Proteins; D021382:Protein Sorting Signals