![]() ![]() Hsp70s and the small Hsps, on the other hand, adopt modular “clamps” for protecting extended hydrophobic structures in their targets. The Hsp60s adopt a barrel-like Anfinsen cage structure for sequestered folding of target proteins. Aside from their differences in size, the structures of these different classes are quite divergent. Further experimentation revealed several types of functions for different chaperone proteins, which may be attributed to the diversity of their structures.Ĭurrent structural information divides the chaperones into five major classes based on their observed molecular weights: Hsp60, Hsp70, Hsp90, Hsp104, and the small Hsps. ![]() The functions of these proteins were validated with the generation of recombinant versions of the proteins that performed their expected functions outside the original cellular environment. Expression of these proteins was increased with heat shock treatment, leading to their label as heat shock proteins (Hsps). These genes are bacterial homologs of Hsp60, Hsp10, Hsp70, Hsp40, and the nucleotide exchange factor for the Hsp70/Hsp40 machine. The disruption of groEL, groES, dnaK, dnaJ, and grpE was found to have deleterious effects for the growth of the bacteriophage. The first of these chaperone proteins was found by Sternberg in 1973 in studies of mutations that disrupted bacteriophage λ head formation. ![]() Ĭonsidering the dense population of the cytosol (average protein conc: 150 mg/mL), Finka and Goloubinoff proposed an inherent need to protect nascent polypeptides from “unwanted associations” that prevent the attainment of the functional protein fold. While this simple and elegant principle governs most biological systems, literature from both the distant and recent past have cited complications in the cellular environment that may disrupt the flow of genetic information. The central dogma of molecular biology states that genes are transcribed into messenger RNAs, which are then translated into the proteins that carry out cellular functions. The structures of domains and the associated functions are discussed in order to illustrate the rationale for the proposed unfoldase function. When possible, it discusses the complete structures for these proteins, and the types of molecular machines to which they have been assigned. It reviews the currently available molecular structures in the Protein Data Bank for several classes of Hsps (Hsp60, Hsp70, Hsp90, and Hsp104). This current article focuses on the resolved structural bases for these functions. Onto this is added specializations that allow the different family members to perform various cellular functions. The term “unfoldases” has been proposed, as this basic function is shared by most members of this protein family. However, neither label encompasses the breadth of these proteins’ functional capabilities. These chaperone proteins also increased in expression as a response to heat shock, hence their label as heat shock proteins (Hsps). These proteins’ ability to prevent unwanted associations led to their being called chaperones. This review summarizes our current understanding of how these proteins function in plants, with a major focus on those systems where the most detailed mechanistic data are available, or where features of the chaperone/foldase system or substrate proteins are unique to plants.Thirty years ago a class of proteins was found to prevent the aggregation of Rubisco. Thus, these diverse proteins affect an exceptionally broad array of cellular processes required for both normal cell function and survival of stress conditions. Different chaperone and foldase systems are required for synthesis, targeting, maturation and degradation of proteins in all cellular compartments. Recent research indicates that many, if not all, cellular proteins interact with chaperones and/or foldases during their lifetime in the cell. The best understood chaperone systems are HSP70/DnaK and HSP60/GroE, but considerable data support a chaperone role for other proteins, including HSP100, HSP90, small HSPs and calnexin. Molecular chaperones are a diverse group of proteins, but they share the property that they bind substrate proteins that are in unstable, non-native structural states. Foldases include protein disulfide isomerase and peptidyl prolyl isomerase, which catalyze the rearrangement of disulfide bonds or isomerization of peptide bonds around Pro residues, respectively. Protein folding in vivo is mediated by an array of proteins that act either as 'foldases' or 'molecular chaperones'. ![]()
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