Subunits and Assembly

In common with the polypeptides of Type I chaperonins, each subunit of Type II chaperonins is organized into three recognizable domains an equatorial domain that contains the ATP-binding site and forms the inter-ring contacts, an apical domain that contains the sites of interaction with target protein, and an intermediate domain that forms a hingelike connection between the two (Ditzel et al, 1998 Klumpp et al., 1997  

Waldmann et al., 1995). The extent of amino acid sequence identity between Type I and Type II chaperonins is only 15—25%, implying a very early divergence from a common ancestor occurring more than two billion years ago (Gutsche et al., 1999 Kubota et al., 1994 Kubota etal., 1995a). 

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Jaenicke, R. Protein folding local structures, domains, subunits, and assemblies. Biochemistry 30, 3147-3161 (1991). 

Fig. 18. Subunits and assembly of integrins. Seven 3 subunits and fourteen a subunits are shown which can form in various combinations to produce dimeric, non-covalently bonded complexes with various ligand binding properties, some of which are indicated in boxes. Abbreviations Fn, fibronectin Lm, laminin coll, collagen Fb, fibrinogen Op, osteopontin ICAM, ICAM family 1 or 2 BM, unidentified basement membrane component(s). a, is also called pl50 or GDI Ic Ol and are GDI la and GDI lb, respectively 32 is GDIS in the immunological literature.

Yeast cytochrome oxidase is built up from seven different subunits, of which four are synthesized in the cytosol, and three in the mitochondria (25). R. 0. Poyton (26) recently reported evidence suggesting that the four cytosolic subunits are synthesized in one peptide chain. This precursor protein might be split by limited proteolysis in connection with or after transport from the cytosol to the inner mitochondrial membrane. Here seems to be an example of a proteolytic process in yeast participating in subcellular translocation of subunits and assembly of an enzyme (see Table V). V/hich proteinase is involved in this process, and whether specific inhibitors also play a part, is as present unknown. 

Jaenicke, R., 1991, Protein Folding Local structures, Domains, Subunits and assemblies. Biochemistry 30 3161-3169. 

The 3 subunits ((31 -(34) are membrane proteins with a single transmembrane domain and an extracellular immunoglobulin-like motif, and perform the regulatory roles of the sodium channel. The (31 subunit accelerates the activation and inactivation kinetics. The (32 subunit is covalently linked to the a subunit, and is necessary for the efficient assembly of the channel. The more recently identified (33 subunit is homologous to (31, but differs in its distribution within the brain and in a weaker accelerating property. The (34 subunit is similar to (32 and is covalently linked to the a subunit. 

Mitochondria are unique organelles in that they contain their own DNA (mtDNA), which, in addition to ribosomal RN A (rRNA) and transfer RN A (tRNA)-coding sequences, also encodes 13 polypeptides which are components of complexes I, III, IV, and V (Anderson et al., 1981). This fact has important implications for both the genetics and the etiology of the respiratory chain disorders. Since mtDNA is maternally-inherited, a defect of a respiratory complex due to a mtDNA deletion would be expected to show a pattern of maternal transmission. However the situation is complicated by the fact that the majority of the polypeptide subunits of complexes I, III, IV, and V, and all subunits of complex II, are encoded by nuclear DNA. A defect in a nuclear-coded subunit of one of the respiratory complexes would be expected to show classic Mendelian inheritance. A further complication exists in that it is now established that some respiratory chain disorders result from defects of communication between nuclear and mitochondrial genomes (Zeviani et al., 1989). Since many mitochondrial proteins are synthesized in the cytosol and require a sophisticated system of posttranslational processing for transport and assembly, it is apparent that a diversity of genetic errors is to be expected. 

Virus maturation and assembly at the cell membrane or the nuclear membrane has long been seen as a potential target for antiviral compounds. For the virus to mature and be released in a conformation that will insure stability and survival of the viral genome in the exttacellular enviromnent, the protein subunits of the capsid or nucle-ocapsids have to be transported to the assembly point where they will form the final particles around the viral nucleic acid. If this process does not occur in an orderly and programmed manner, the capsid subunits will not form the required multimers and the viral components will become targets for the cellular disposal mechanisms. 

Figure 37-2. RNA polymerase (RNAP) catalyzes the polymerization of ribonucleotides into an RNA sequence that is complementary to the template strand of the gene. The RNA transcript has the same polarity (5 to 3 ) as the coding strand but contains L) rather than T. E coli RNAP consists of a core complex of two a subunits and two p subunits (P and p ). The holoen-zyme contains the 0 subunit bound to the ajPP core assembly. The co subunit is not shown. The transcription "bubble" is an approximately 20-bp area of melted DNA, and the entire complex covers 30-75 bp, depending on the conformation of RNAP.

Moore,T and Bennett,A.B (1994) Tomato finit polygalacturonase isozyme-1. Characterisation of the beta-subunit and its state of assembly in vivo. Plant Physiology. 106. 1461-1469. 

In the family of cation pumps, only the Na,K-ATPase and H,K-ATPase possess a p subunit glycoprotein (Table II), while the Ca-ATPase and H-ATPase only consist of an a subunit with close to 1 000 amino acid residues. It is tempting to propose that the p subunit should be involved in binding and transport of potassium, but the functional domains related to catalysis in Na,K-ATPase seem to be contributed exclusively by the a subunit. The functional role of the P subunit is related to biosynthesis, intracellular transport and cell-cell contacts. The P subunit is required for assembly of the aj8 unit in the endoplasmic reticulum [20]. Association with a j8 subunit is required for maturation of the a subunit and for intracellular transport of the xP unit to the plasma membrane. In the jSl-subunit isoform, three disulphide... 

The recent identification of the H,K-ATPase P subunit and the demonstration that the ratio for the H,K-ATPase catalytic a subunit/) subunit is about 1 1 [62,71] might bring new insight into the structural and functional assembly of H,K-ATPase in situ. 

Assembly of nucleic acid and protein subunits (and membrane components in enveloped viruses) into new virus particles ... 

John, D.C.A., Grant, M.E. and Bulleid, N.J. (1993) Cell-free synthesis and assembly of prolyl 4-hydroxylase - the role of the beta-subunit (pdi) in preventing misfolding and aggregation of the alpha-subunit. EMBOJoumal 2,1587-1595. 

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