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Heat Shock Proteins


Introduction:

Heat Shock Proteins (Hsps) are ubiquitously expressed in all living organisms and are essential for cell growth and viability. The expression levels of these highly conserved proteins are upregulated upon exposure to high temperature or other stresses such as altered pH, oxygen deprivation etc. The primary function of these proteins is to govern the folding and refolding mechanism of naive and stress denatured proteins. They dynamically associate with the exposed hydrophobic surfaces of the unfolded polypeptides, prevent their aggregation and thereby, potentiate the folding process. Additionally, Hsps are also involved in diverse cellular roles such as protein assembly, translocation and degradation. Hence these proteins are the essential determinants of protein quality control in cell.

Network
A functional HSP family network required for the maintenance of cellular protein quality control.
[Click on the image to enlarge]

Classification:

Based on their nature of functions and molecular mass, HSPs are classified broadly into six major families namely Hsp70, Hsp40 (J-proteins), Hsp60 (chaperonins), Hsp90, Hsp100 (Clp proteins) and small heat shock proteins (sHsps). They function cooperatively by forming an intricate molecular network (below figure) thereby, maintaining the overall cellular protein homeostasis. A brief introduction for each HSP family has been enlisted below.

Hsp70 and Hsp40/J-Protein family:

Heat shock proteins belonging to 70 kDa (Hsp70s) are the most abundant and well characterized HSPs in bacterial and eukaryotic systems.  Presence of several types of Hsp70s has been reported in all living organisms, and they are typically distributed 1-2 per cellular compartment (1,2). Hsp70s contain two conserved functional domains. The N-terminal Nucleotide binding domain (NBD) and substrate binding domain (SBD) at the C-terminus connected via a highly conserved linker region. The SBD composed of a variable helical region which functions as a ‘lid’ over the peptide binding cavity and regulates the substrate interaction (1,2). The specificity of Hsp70s is largely determined by their partner J-proteins, and together they function as a ‘molecular chaperone machine’ (3). In organisms, the members of J-protein family are found in large numbers and often exceed the members of Hsp70s. Therefore, each Hsp70 is capable of interacting with multiple J-protein partners in connection to various physiological functions, thus forming a complex network of chaperones (2,3).

In order to interact with the highly conserved Hsp70s, all J-proteins contain a ~70 amino acids long signature J-domain that contacts the ATPase domain of Hsp70s.  The J-domain of J-proteins stimulates Hsp70's ATPase activity, thus promoting the interaction of Hsp70 with substrates. Structurally, J-proteins are very diverse and further classified into four subtypes. Type I and II closely resembles to E.coli DnaJ contains N-terminal J-domain followed by a G/F region, Zinc-Finger region (absent in Type II) and C-terminal substrate binding regions (3,4). While, the type III J-proteins are polymorphic in domain organization and contain only a characteristic J-domain anywhere in the sequence. The type IV J-proteins also known as J-like proteins (JLPs) shares overall a similar J-domain structural fold but lacks a signature HPD sequence which is critical for the stimulation of Hsp70’s activity (3,4). As a chaperone machine, Hsp70s and J-proteins are implicated in several essential physiological functions (3,4), including prevention of misfolding of newly synthesized polypeptides, protein translocation across the membranes and degradation processes (5). To know more about Hsp70 [Click here] & Hsp40 [Click here]

Hsp60/Chaperonin family:

Folding of multimeric proteins with complex topologies often require a specialized class of oligomeric folding chambers called chaperonins. Based on structural organization, chaperonins are further classified into two distinct subgroups. Group I chaperonins (GroEL/ES) found in prokaryotes and endosymbiotic organelles such as mitochondria and chloroplast. On the other hand, group II chaperonins (TRiC/CCT) present in archaeal and localized in the cytosolic compartment of eukaryotic systems (2,6). GroEL/ES molecule organized into a double ring structure consists of 14 identical 57 kDa subunits capped with 10 kDa GroES subunit. In contrast, group II chaperonins are hetero-oligomeric ring complexes consists of eight to nine subunits per ring with a built in apical domain which serves as a lid over the central cavity (6). However, both groups of chaperonins share a common subunit organization which consists of equatorial domain, followed by central hinge-region and an apical domain. The chaperonin secret chamber provides an optimum microenvironment to facilitate the productive protein folding into the native state in an ATP dependent cooperative process (2,7).[Know more...]

Hsp90 Family:

Hsp90 is an one abundant cellular chaperone essential for folding of a vast array of client proteins, including protein kinases and transcription factors (7), which are involved in the signal transduction process. Typically, they are distributed one to two copies per eukaryotic cells localized either in the cytoplasm, endoplasmic reticulum (Grp94) and mitochondrial compartments (TRAP1). Like other classes of HSPs, the levels of Hsp90 are upregulated into several folds upon induction to stress in order to protect the conformational flexibility of its client proteins (8).  All the Hsp90 family members from bacteria (HtpG) to eukaryotic (Grp94) organisms share a similar modular protein architecture and mode of action. Hsp90 contains a conserved N-terminal 'ATP binding domain' followed by ‘middle region’ and C-terminal dimerizing domain (8), which are involved  in the binding to different cochaperones and client proteins. They form a constitutive homodimer and in the presence of ATP-bound state adopt a circular structure to stabilize the conformational plasticity of client proteins and assist in the maturation into their native states (8). [Know more...]

Hsp100 Family:

The reactivation of misfolded proteins and the clearance of irreversible protein aggregates require a specialized 'remodeling machines' belonging to Hsp100 class of chaperones. They constitute AAA+ superfamily of ATPases and have the ability to solubilize almost all kinds of aggregated proteins in an ATP dependent manner (9,10). They predominantly present in large numbers in the prokaryotes as compared to eukaryotes systems where it is localized either one to two copies in the cytoplasm or mitochondrial compartments (9,10). Hsp100 chaperones are broadly categorized into two classes. The Class I proteins are with two highly homologous AAA+ modules such as Hsp104, bacterial ClpB and their distant relatives ClpA, ClpC, and class II chaperones containing one ATP-binding domain like ClpX and HslU (10). The protein forms a two-tier hexameric ring shaped structures. Binding of ATP to the N-terminal domain stabilizes its oligomeric state and interactions with the substrate (10). Under in vivo condition both ClpB and Hsp70 systems act synergistically in the disaggregating process. In addition to protein remodeling function, Hsp100 proteins like ClpX target dead aggregates to proteasomal degradation via its interaction with cofactor ClpP, and therefore prevent nucleation of inactive complexes that lead to pathological conditions (9). [Know more...]

Small Heat Shock Protein family (sHsps):

Phylogenitically, sHsps are widespread and found throughout in all kingdoms.  Normally, they are present in 1-2 per archaeal species while, distributed in large numbers in different cellular compartments, including mitochondria, endoplasmic reticulum and chloroplast of higher eukaryotes. The levels of sHsps are upregulated upon stress together with members of other HSP families (11).  They are diverse in size and usually contain a molecular mass ranging from 12-43 kDa. The small Hsp monomers contain a highly conserved α-crystallin domain with protein sequence ranges from 80-100 amino acids in length flanked by hypervariable N-terminal region and C-terminal extension (12). Upon overexpression, sHsps forms an active sub complex to assemble a poly disperse oligomeric structures thus creating a basin with large surface area to bind aggregation prone substrates. They protect non-native proteins from irreversible aggregation in the absence of ATP and subsequently transfer them to another HSP family machinery to ensure the proper folding. The sHsps plays a critical role in cellular protein homeostasis and several other physiological functions such as thermotolerance, differentiation and signal transduction (11).[Know more...]