Polyglutamine aggregates alter protein folding homeostasis in Caenorhabditis elegans.

Expansion of polyglutamine repeats in several unrelated proteins causes neurodegenerative diseases with distinct but related pathologies. To provide a model system for investigating common pathogenic features, we have examined the behavior of polyglutamine expansions expressed in Caenorhabditis elegans. The expression of polyglutamine repeats as green fluorescent protein (GFP)-fusion proteins in body wall muscle cells causes discrete cytoplasmic aggregates that appear early in embryogenesis and correlates with a delay in larval to adult development. The heat shock response is activated idiosyncratically in individual cells in a polyglutamine length-dependent fashion. The toxic effect of polyglutamine expression and the formation of aggregates can be reversed by coexpression of the yeast chaperone Hsp104. The altered homeostasis associated with polyglutamine aggregates causes both the sequestration of an otherwise soluble protein with shorter arrays of glutamine repeats and the relocalization of a nuclear glutamine-rich protein. These observations of induced aggregation and relocalization have implications for disorders involving protein aggregation.

Negative regulation of the heat shock transcriptional response by HSBP1.

In response to stress, heat shock factor 1 (HSF1) acquires rapid DNA binding and transient transcriptional activity while undergoing conformational transition from an inert non-DNA-binding monomer to active functional trimers. Attenuation of the inducible transcriptional response occurs during heat shock or upon recovery at non-stress conditions and involves dissociation of the HSF1 trimer and loss of activity. We have used the hydrophobic repeats of the HSF1 trimerization domain in the yeast two-hybrid protein interaction assay to identify heat shock factor binding protein 1 (HSBP1), a novel, conserved, 76-amino-acid protein that contains two extended arrays of hydrophobic repeats that interact with the HSF1 heptad repeats. HSBP1 is nuclear-localized and interacts in vivo with the active trimeric state of HSF1 that appears during heat shock. During attenuation of HSF1 to the inert monomer, HSBP1 associates with Hsp70. HSBP1 negatively affects HSF1 DNA-binding activity, and overexpression of HSBP1 in mammalian cells represses the transactivation activity of HSF1. To establish a biological role for HSBP1, the homologous Caenorhabditis elegans protein was overexpressed in body wall muscle cells and was shown to block activation of the heat shock response from a heat shock promoter-reporter construct. Alteration in the level of HSBP1 expression in C. elegans has severe effects on survival of the animals after thermal and chemical stress, consistent with a role for HSBP1 as a negative regulator of the heat shock response.