An essential connection: link between Hsp70's domains at last

Communication between the ATPase and substrate binding domains of Hsp70 is critical for regulated interaction between this molecular chaperone and its client proteins. In this issue of Molecular Cell, Jiang et al. (2005) report the structure of an intact Hsp70, revealing critical interactions between the two domains.     

Chaperone receptors: guiding proteins to intracellular compartments

Despite mitochondria and chloroplasts having their own genome, 99% of mitochondrial proteins (Rehling et al., Nat Rev Mol Cell Biol 5:519–530, 2004) and more than 95% of chloroplast proteins (Soll, Curr Opin Plant Biol 5:529–535, 2002) are encoded by nuclear DNA, synthesised in the cytosol and imported post-translationally. Protein targeting to these organelles depends on cytosolic targeting factors, which bind to the precursor, and then interact with membrane receptors to deliver the precursor into a translocase. The molecular chaperones Hsp70 and Hsp90 have been widely implicated in protein targeting to mitochondria and chloroplasts, and receptors capable of recognising these chaperones have been identified at the surface of both these organelles (Schlegel et al., Mol Biol Evol 24:2763–2774, 2007). The role of these chaperone receptors is not fully understood, but they have been shown to increase the efficiency of protein targeting (Young et al., Cell 112:41–50, 2003; Qbadou et al., EMBO J 25:1836–1847, 2006). Whether these receptors contribute to the specificity of targeting is less clear. A class of chaperone receptors bearing tetratricopeptide repeat domains is able to specifically bind the highly conserved C terminus of Hsp70 and/or Hsp90. Interestingly, at least of one these chaperone receptors can be found on each organelle (Schlegel et al., Mol Biol Evol 24:2763–2774, 2007), which suggests a universal role in protein targeting for these chaperone receptors. This review will investigate the role that chaperone receptors play in targeting efficiency and specificity, as well as examining recent in silico approaches to find novel chaperone receptors.     

Transcriptional ShortCUTs

Recent work from Kuehner and Brow (2008) and Thiebaut et al. (2008) in Molecular Cell and Jenks et al. (2008) in Molecular and Cellular Biology reveals that regulated expression of central nucleotide synthesis pathway components directs start site-dependent RNA polymerase II termination.     

A ubiquitin-like protein unleashes ubiquitin ligases

Modification of cullin-RING ubiquitin ligases by the ubiquitin-like molecule Nedd8 promotes substrate ubiquitination. A crystal structure of a cullin modified by Nedd8 recently reported in Cell (Duda et al., 2008) and a biochemical study in Molecular Cell (Saha and Deshaies, 2008) reveal the dramatic impact on the ligase machinery by conjugation of ubiquitin or ubiquitin-like proteins.     

Yos9p: a sweet-toothed bouncer of the secretory pathway

In this issue of Molecular Cell, Bhamidipati et al., (2005) and Kim et al., (2005), and Szathmary et al. (2005), and demonstrate that Yos9p selectively binds to aberrant glycoproteins in the endoplasmic reticulum (ER) and targets them for destruction through the ER-associated protein degradation pathway.     

The release of Hsp70 from A431 carcinoma cells is mediated by secretory-like granules

In our earlier work we have demonstrated that the treatment of squamous carcinoma cell line A431 with a pharmacological inhibitor of phospholipase C activity, U73122, resulted in fast release of stress-inducible heat shock protein 70 (Hsp70) into the extracellular medium (Evdonin et al., Cancer Cell Int., 4, 2, 2004). The purpose of the present study was to identify cellular organelles involved in the release of Hsp70 from A431 cells. We determined that Hsp70 is present in granules located at the periphery of cells, which had been treated with U73122 or subjected to heat shock. An inhibitor of the classical protein export pathway, brefeldin A was found to prevent the U73122-induced appearance of Hsp70 in the extracellular medium and in the peripheral granules. These findings suggest that vesicular transport is involved in Hsp70 release. The Hsp70-containing granules did not carry markers specific for lipid bodies, endosomes, or lysosomes. However, they were positive for a marker of secretory granules, i.e. chromogranin A. The levels of extracellular Hsp70 and chromogranin A were found to increase simultaneously. The secretory-like granule-dependent transport of Hsp70 was also studied in minimally transformed human HaCaT keratinocytes. We found that after U73122 and heat stress treatment, HaCaT cells secreted Hsp70 in a manner similar to A431 cells. Collectively our results suggest that human keratinocyte-derived cells release Hsp70 in the extracellular medium through a pathway involving secretory-like granules.     

A Grp on the Hsp90 mechanism

In a recent issue of Molecular Cell, Dollins et al. (2007) present the crystal structure of Grp94, which highlights the similarity between Grp94 and Hsp90 and provides insight into the resting state of Grp94 and potentially other Hsp90 family members.     

A network of its own: the unique interactome of the Hsp90 cochaperone, Sba1/p23

In this issue of Molecular Cell, Echtenkamp et?al. (2011) show that the molecular chaperone Sba1/p23, thought to function primarily as a key modulator of the Hsp90 chaperone complex, also operates in its own sphere of influence outside of its obligations to Hsp90.     

GU-rich RNA: expanding CUGBP1 function, broadening mRNA turnover

In this issue of Molecular Cell, Vlasova et al. (2008) identify the GU-rich element (GRE) as a novel, widespread, degradation-promoting sequence through which the RNA-binding protein CUGBP1 elicits mRNA decay.     

Hit and run: transient deubiquitylase activity in a chromatin-remodeling complex

In this issue of Molecular Cell, Conaway and colleagues (Yao et al., 2008) provide a glimpse into an interesting mechanism to control a deubiquitylating enzyme via interaction of two complexes-the 19S proteasome regulatory particle and an ATP-dependent nucleosome remodeling complex.     

Tbf1 or not Tbf1?

In this issue of Molecular Cell, Hogues et al. (2008) demonstrate a wholesale shift in the key regulatory protein involved in ribosomal protein (RP) synthesis during the evolution of S. cerevisiae and, en passant, raise interesting questions about the relationship between RP genes and telomeres.     

Connecting mitochondria and innate immunity

Viral infection results in the activation of multiple signaling pathways, but how these pathways are coordinated remains a mystery. Two studies, one published in this issue of Cell (Seth et al., 2005) and the other in Molecular Cell (Xu et al., 2005), identify a new intracellular signaling protein that is required for activating type I interferon expression in response to viral infection. In addition,Seth et al. (2005) show that the function of this protein, which they call MAVS, requires that it be localized to the mitochondria. This observation establishes an unexpected link between innate immunity and an organelle with evolutionary origins in aerobic bacteria.     

Nucleotide Binding by Lhs1p Is Essential for Its Nucleotide Exchange Activity and for Function in Vivo

Protein translocation and folding in the endoplasmic reticulum of Saccharomyces cerevisiae involves two distinct Hsp70 chaperones, Lhs1p and Kar2p. Both proteins have the characteristic domain structure of the Hsp70 family consisting of a conserved N-terminal nucleotide binding domain and a C-terminal substrate binding domain. Kar2p is a canonical Hsp70 whose substrate binding activity is regulated by cochaperones that promote either ATP hydrolysis or nucleotide exchange. Lhs1p is a member of the Grp170/Lhs1p subfamily of Hsp70s and was previously shown to function as a nucleotide exchange factor (NEF) for Kar2p. Here we show that in addition to this NEF activity, Lhs1p can function as a holdase that prevents protein aggregation in vitro. Analysis of the nucleotide requirement of these functions demonstrates that nucleotide binding to Lhs1p stimulates the interaction with Kar2p and is essential for NEF activity. In contrast, Lhs1p holdase activity is nucleotide-independent and unaffected by mutations that interfere with ATP binding and NEF activity. In vivo, these mutants show severe protein translocation defects and are unable to support growth despite the presence of a second Kar2p-specific NEF, Sil1p. Thus, Lhs1p-dependent nucleotide exchange activity is vital for ER protein biogenesis in vivo.     

Structural basis for the cooperation of Hsp70 and Hsp110 chaperones in protein folding

Protein folding by Hsp70 is tightly controlled by cochaperones, including J-domain proteins that trigger ATP hydrolysis and nucleotide exchange factors (NEFs) that remove ADP from Hsp70. Here we present the crystal structure of the yeast NEF Sse1p (Hsp110) in complex with the nucleotide-binding domain (NBD) of Hsp70. Hsp110 proteins are homologous to Hsp70s and consist of an NBD, a beta sandwich domain, and a three helix bundle domain (3HBD). In the complex, the NBD of Sse1p is ATP bound, and together with the 3HBD it embraces the NBD of Hsp70, inducing opening and the release of bound ADP from Hsp70. Mutations that abolish NEF activity are lethal, thus defining nucleotide exchange on Hsp70 as an essential function of Sse1p. Our data suggest that Sse1p does not employ the nucleotide-dependent allostery and peptide-binding mode of canonical Hsp70s, and that direct interactions of substrate with Sse1p may support Hsp70-assisted protein folding in a cooperative process.     

If the prophet does not come to the mountain: dynamics of signaling complexes in NF-kappaB activation

Recruitment of the NF-kappaB-activating IKK signaling complex to the TNF receptor is shown to be driven by induced binding of NEMO, a regulatory component of this complex, to K63-linked polyubiquitin chains attached to RIP1, a receptor-associated adaptor protein (Ea et al., 2006 [in a recent issue of Molecular Cell]; Li et al., 2006; Wu et al., 2006a).     

Distinct but overlapping functions of Hsp70, Hsp90, and an Hsp70 nucleotide exchange factor during protein biogenesis in yeast

Hsp70 and Hsp90 molecular chaperones play essential roles in protein expression and maturation, and while catalyzing protein folding they can "decide" to target mis-folded substrates for degradation. In this report, we show for the first time distinct but partially overlapping requirements for Hsp90, Hsp70, and an Hsp70 nucleotide exchange factor (NEF) at different steps during the biogenesis of a model substrate, firefly luciferase (FFLux), in yeast. By examining the inducible expression of FFLux in wild type cells and in specific yeast mutants, we find that the Fes1p NEF is required for efficient FFLux folding, whereas the Hsp70, Ssa1p, is required for both protein folding and stability, and to maintain maximal FFLux mRNA levels. In contrast, Hsp90 function was primarily necessary to express the FFLux-encoding gene from an inducible promoter. Together, these data indicate previously unknown roles for these proteins and point to the complexity with which chaperones and cochaperones function in the cell.