, formed by the aggregated states of the cytosolic proteins Sup35, Rnq1, and Ure2, respectively (1). Sup35 is a translation termination factor; Ure2 is a regulator that acts to repress transcription of a set of genes involved in nitrogen catabolism; the function of Rnq1 is unknown. Prion proteins can form different conformational states resulting in prion strains having different heritable traits. For propagation in the cell population, physical transmission of the prion template, often referred to as the propagon or seed, is required to allow conversion of newly synthesized protein to the prion conformation (2).Somewhat paradoxically, the propagation of yeast prions appears to be inexorably reliant on the function of molecular chaperones, proteins that normally function to prevent protein misfolding (2). Two chaperone systems have been linked to prion propagation: the hexameric AAAϩ ATPase Hsp104 and the J-protein (Hsp40):Hsp70 chaperone machinery, with its associated nucleotide exchange factors (3). Hsp104, like its ortholog ClpB, functions in protein remodeling by threading partially folded proteins through its central pore and is stringently required for the propagation of all identified yeast prions (1,4,5).Hsp70s function with their obligate cochaperones, J-proteins, which act to stimulate Hsp70 ATPase activity and stabilize their interaction with client proteins (6). Although J-proteins are very diverse in sequence and structure, they possess a highly conserved J-domain that is responsible for the stimulation of the ATPase activity of Hsp70s. One cytosolic J-protein, Sis1, is required for propagation of [RNQ ϩ ] (7). In addition, multiple individual amino acid substitutions in the cytosolic Hsp70s Ssa1/2 that impair propagation of [PSI ϩ propagation implies an involvement of an unidentified J-protein as well.The currently favored model for prion propagation posits chaperone-mediated fragmentation of prion complexes to produce sufficient prion seeds to assure consistent transmission of seeds to daughter cells, thus maintaining the prion in the cell population (2,(10)(11)(12)(13)(14). Supporting this model, inhibition of Hsp104 activity results in an increase in the size of Sup35 and Rnq1 prion complexes and subsequent prion loss, which has been shown in the case of [PSI ϩ ] to be dependent on cell division (10,11,15,16). Additional support for this idea comes from reports of fragmentation of prion fibers in vitro by Hsp104 (17) and the apparent decrease in the number of [PSI ϩ ] prion seeds in cells expressing a dominant mutation in the Hsp70 SSA1 gene (8). An increase in the size of Rnq1 polymers, followed by [RNQ ϩ ] loss, also occurs upon depletion of Sis1, a partner of Ssa1 (15). Together these data suggest cooperation between the 2 chaperone systems. Such cooperation has precedent, as Hsp104 is known to function in disaggregation of amorphous protein aggregates in conjunction with J-protein:Hsp70 chaperone machinery, with J-protein/Hsp70 and Hsp104 machineries acting sequentially (4).The yeast cytos...

J proteins are obligate cochaperones of Hsp70s, stimulating their ATPase activity and thus allowing them to function in multiple cellular processes. In most cellular compartments, an Hsp70 works with multiple, structurally divergent J proteins. To better understand the functional specificity of J proteins and the complexity of the Hsp70:J protein network, we undertook a comprehensive analysis of 13 J proteins of the cytosol of the yeast Saccharomyces cerevisiae. Phenotypes caused by the absence of four proteins, Sis1, Jjj1, Jjj3, and Cwc23, could not be rescued by overexpression of any other cytosolic J protein, demonstrating the distinctive nature of J proteins. In one case, that of Zuo1, the phenotypic effects of the absence of a J protein could be rescued by overexpression of only one other J protein, Jjj1, which, like Zuo1, is ribosome-associated. In contrast, the severe growth phenotype caused by the absence of the cytosol's most abundant J protein, Ydj1, was substantially rescued by expression of J domaincontaining fragments of many cytosolic J proteins. We conclude that many functions of Hsp70 chaperone machineries only require stimulation of Hsp70's ATPase activity by J protein partners. However, a subset of Hsp70 functions requires specific J protein partners, likely demanding either sublocalization within the compartment or binding to specific client proteins.Hsp40 ͉ Hsp70 ͉ molecular chaperone ͉ multigene family T hrough their action in protein folding, degradation, translocation across membranes, and disassembly of protein complexes, molecular chaperones are important participants in many crucial cellular processes (1, 2). Hsp70s and their J protein partners (at times referred to as Hsp40s) constitute an important component of the cellular ''chaperone'' in virtually all living systems (3). In most cellular compartments, an Hsp70 has multiple J protein partners. The cytosol, a hub of activity for a variety of crucial cellular processes, is no exception. With a goal of better understanding the degree of functional overlap among J proteins, as well as their individual specificities, we choose the cytosol of Saccharomyces cerevisiae as the focus of a comprehensive analysis of J protein function.Although J proteins are obligate partners of Hsp70s, it is the Hsp70 that has long been considered the core of the Hsp70:J protein chaperone machine by virtue of their client protein interaction, which is modulated by nucleotide binding (4). ATP binding fosters rapid binding to client proteins, whereas nucleotide hydrolysis stabilizes the interaction. The cytosol of S. cerevisiae contains two predominant classes of Hsp70s, Ssa (SSA1-4) and Ssb (SSB1-2) (5). Yeast cells expressing only one representative of a class grow very similarly to wild-type cells under a variety of conditions. But the classes are functionally distinct, because a member of the Ssa class cannot substitute for an Ssb Hsp70 and vice versa (6).A role of all J proteins is stimulation of the ATPase activity of their partner Hsp70s. Such stimulation...

Yeast prions are protein-based genetic elements capable of self-perpetuation. One such prion, [RNQ(+)], requires the J-protein Sis1, an Ssa Hsp70 co-chaperone, as well as the AAA+ ATPase, Hsp104, for its propagation. We report that, upon depletion of Sis1, as well as upon inactivation of Hsp104, Rnq1 aggregates increased in size. Subsequently, cells having large aggregates, as well as an apparently soluble pool of Rnq1, became predominant in the cell population. Newly synthesized Rnq1 localized to both aggregates and bulk cytosol, suggesting that nascent Rnq1 partitioned into pools of prion and nonprion conformations, and implying that these large aggregates were still active as seeds. Ultimately, soluble Rnq1 predominated, and the prion was lost from the population. Our data suggest a model in which J-protein:Hsp70 machinery functions in prion propagation, in conjunction with Hsp104. Together, these chaperones facilitate fragmentation of prion polymers, generating a sufficient number of seeds to allow efficient conversion of newly synthesized Rnq1 into the prion conformation.

Hsp70 chaperone systems are very versatile machines present in nearly all living organisms and in nearly all intracellular compartments. They function in many fundamental processes through their facilitation of protein (re)folding, trafficking, remodeling, disaggregation, and degradation. Hsp70 machines are regulated by co-chaperones. J-domain containing proteins (JDPs) are the largest family of Hsp70 co-chaperones and play a determining role functionally specifying and directing Hsp70 functions. Many features of JDPs are not understood; however, a number of JDP experts gathered at a recent CSSI-sponsored workshop in Gdansk (Poland) to discuss various aspects of J-domain protein function, evolution, and structure. In this report, we present the main findings and the consensus reached to help direct future developments in the field of Hsp70 research.

Significant progress has been made in unraveling the molecular biology of rice in the past two decades. Today, rice stands as a forerunner amongst the cereals in terms of details known on its genetics. Evidence show that salt tolerance in plants is a quantitative trait. Several traditional cultivars, landraces, and wild types of rice like Pokkali, CSR types, and Porteresia coarctata appear as promising materials for donation of requisite salt tolerance genes. A large number of quantitative trait loci (QTL) have been identified for salt tolerance in rice through generation of recombinant inbred lines and are being mapped using different types of DNA markers. Salt-tolerant transgenic rice plants have been produced using a host of different genes and transcript profiling by micro- and macroarray-based methods has opened the gates for the discovery of novel salt stress mechanisms in rice, and comparative genomics is turning out to be a critical input in this respect. In this paper, we present a comprehensive review of the genetic, molecular biology, and comparative genomics effort towards the generation of salt-tolerant rice. From the data on comprehensive transcript expression profiling of clones representing salt-stress-associated genes of rice, it is shown that transcriptional and translational machineries are important determinants in controlling salt stress response, and gene expression response in tolerant and susceptible rice plants differs mainly in quantitative terms.