SUMMARY PARAGRAPH for SSA4
SSA1, SSA2, SSA3, and SSA4 encode chaperone proteins that comprise the S. cerevisiae SSA subfamily of cytosolic HSP70 proteins (1). HSP70 is a large family of proteins that has been evolutionarily conserved from bacteria (DnaK) to humans (HSP72/73). HSP70 proteins were originally classified based upon their induction by heat shock and their size of ~70kDa. The main function of these proteins is to serve as molecular chaperones, binding newly-translated proteins to assist in proper folding and prevent aggregation/misfolding (reviewed in 6 and 7). In yeast, HSP70s are also involved in disassembling aggregates of misfolded proteins, translocating select proteins into the mitochondria and ER, degrading aberrant proteins, and regulating the expression of other heat shock proteins (8, 9, 10, 11, and reviewed in 12, 7, and 6). S. cerevisiae has at least 9 cytosolic forms of HSP70 (SSA1, SSA2, SSA3, SSA4, SSB1, SSB2, SSE1, SSE2, SSZ1), 2 HSP70s which are found in the endoplasmic reticulum (KAR2, LHS1), and 3 mitochondrial HSP70s (SSC1, SSQ1, ECM10).
The 4 genes of the SSA subfamily are closely related, with Ssa4p sharing 85% amino acid identity with Ssa1p and Ssa2p and 90% identity with Ssa3p (13). SSA4 null mutants are viable and have no phenotype that is distinguishable from wild-type yeast (1). SSA4 expression is not detectable under normal growth conditions, but is upregulated upon heat shock, cold and ethanol stress, and during diauxic shift (3, 14, 15, 16). Increased expression after heat shock is mediated by the transcriptional activator Hsf1p, which recognizes and binds a heat shock element in the SSA4 promoter (17, 3). In ssa1ssa2 double null mutant cells, SSA4 is highly expressed, even under non-inducing conditions, and is able to sustain growth (1). In starved or ethanol-stressed cells, Ssa4p accumulates in the nucleus, a process which is mediated by a hydrophobic stretch in the N-terminal domain of Ssa4p and the beta-importin Nmd5p (4, 15).
Most of the structural knowledge of the S. cerevisiae HSP70 proteins is based on experimental evidence from bacterial DnaK, mammalian HSP70, and Ssa1p (18 and reviewed in 6). All Hsp70s contain an N-terminal ATPase domain and a C-terminal peptide binding domain. ATPase activity of HSP70s is intrinsically weak but can be enhanced by interaction with DnaJ/HSP40 proteins (reviewed in 6). It has been shown for Ssa1p, and based on similarity is implicated for the remaining Ssa subfamily, that activity is stimulated by interaction with the DnaJ/HSP40 co-chaperones Ydj1p, Sis1p, Sti1p, and Cns1p (2, 19, 20, 21). Substrate binding is regulated by ATP turnover; in the presence of ATP, peptide exchange is rapid and the binding constant is low while when ADP is bound, peptide exchange is slower and the substrate affinity higher (reviewed in 6). The rate of Ssa protein ATP/ADP exchange is stimulated by the nucleotide exchange factors Fes1p and Snl1p (22, 23).
The effect of SSA4 expression has also been studied in yeast models of human disease, such as the prion disease Creutzfeldt-Jakob disease (OMIM), Huntington disease (OMIM), and Parkinson disease (OMIM). In cells carrying the yeast prion [PSI+] (isoform of Sup35p), excess Ssa protein promotes prion formation and propagation and also inhibits the curing affect of Hsp104p (24). In contrast, overexpression of SSA4 counteracts polyglutamine aggregation and toxicity, which are two hallmarks of Huntington disease, that are observed in yeast cells propagating the [PIN+] prion form of Rnq1p (25). SSA4 has also been implicated in protecting S. cerevisiae cells that express human alpha-synuclein, the protein that forms amyloid fibers in Parkinson disease, from apoptosis (26).
Last updated: 2006-02-06