Core subunits

Chaperones. Figure 2 The multiple roles of BiP in the biogenesis of the secretory proteins. BiP, immunoglobulin heavy chain binding protein ER, endoplasmic reticulum ERAD, ER-associated degradation ERj, resident ER protein with J-domain Sec61, core subunit of the protein translocase UPR, unfolded protein response that involves several signal transduction pathways that are activated in order to increase the biosynthetic capacity and decrease the biosynthetic burden of the ER... 

Greiner, D.P., Hughes, K.A., Gunasekera, A.H., and Meares, C.F. (1996) Binding of the sigma-70 protein to the core subunits of Escherichia coli RNA polymerase, studied by iron-EDTA protein footprinting. Proc. Natl. Acad. Sci. USA 93, 71-75. 

Sarnowski, T.J., Swiezewski, S., Pawlikowska, K., Kaczanowski, S., and Jerzmanowski, A. (2002) AtSWBB, an Arabidopsis homolog of SWI3, a core subunit of yeast Swi/Snf chromatin remodeling complex, interacts with FCA, a regulator of flowering time. Nucleic Acids Res. 30, 3412-3421. 

The Protein Coat. Twenty-four polypeptides assemble into a hollow sphere, of ca. 100-120 X in outer diameter, to form the protein coat of ferritin. The diameter of the interior, which becomes filled with hydrous ferric oxide, is ca. 70-80 A. Subunit assembly appears to be spontaneous the coat remains assembled even without the iron core. Subunit biosynthesis is actually controlled by the amount of iron to be stored by a cell the subunit templates (mRNAs) are stored in the cytoplasm of a cell in a repressed form and are recruited for biosynthesis when the concentration of iron increases (3). 

Tetranuclear iron-sulfur clusters are key relay stations in the electron flow in photosynthesis. Photosystem I comprises three subunits, PsaA, PsaB and PsaC. The latter contains two [Fe4S4] centres FA and FB. The core subunits PsaA and B, respectively, house a [Fe4S4] centre denoted FX in addition to other, organic cofactors. The role of this latter cluster was probed in preparations partially devoid of PsaC. It was concluded from the results that FX has a major role in controlling the electron transport through PS I.236 Since the final acceptor of the electrons in PS I is a ferredoxin with a [Fe2S2] cluster it was of interest to study a... 

Electron microscopy of the core dihydrolipoyl transacylase from E. coli reveals a striking octahedral symmetry which has been confirmed by X-ray diffraction.306 3073 The core from pyruvate dehydrogenase has a mass of 2390 kDa and contains 24 identical 99.5-kDa E2 subunits. The 2-oxoglutarate dehydrogenase from E. coli has a similar but slightly less symmetric structure. Each core subunit is composed of three domains. A lipoyl group is bound in amide linkage to lysine 42 and protrudes from one end of the domain. 

Figure 15-15 Sequence of reactions catalyzed by a-oxoacid dehydrogenases. The substrate and product are shown in boxes, and the path of the oxidized oxoacid is traced by the heavy arrows. The lipoic acid "head" is shown rotating about the point of attachment to a core subunit. However, a whole flexible domain of the core is also thought to move.

Figure 18-10 Structure of mitochondrial cytochrome c oxidase. (A) Stereoscopic Ca backbone trace for one monomeric complex of the core subunits I, II, and III. (B) Stereoscopic view showing all 13 subunits. The complete complex is a dimer of this structure. From Tsukihara et al.125 (C) MolScript ribbon drawing of one monomeric unit. The horizontal lines are drawn at distances of + 1.0 and +2.0 nm from the center of the membrane bilayer as estimated from eight phospholipid molecules bound in the structure. From Wallin et al.127 Courtesy of Arne Elofsson.

This was the first complete structure of the bc complex. The structure provided information about all 11 subunits and revealed that subunit 9, the mitochondrial targeting presequence of ISP, exists between two core subunits, which are most likely a mitochondrial targeting presequence peptidase. We have solved the structures of the bc complex in two different crystal forms. Surprisingly, the conformation of the Rieske FeS protein was totally different between two crystal forms, and this provided a crucial insight of the electron bifurcation mechanism at the Qp site (see Section II,F). 

The final stage of the light reactions is catalyzed by photosystem 1 (Figure 19.19). The core of this system is a pair of similar subunits psaA (83 kd) and psaB (82 kd). These subunits are quite a bit larger than the core subunits of photosystem II and the bacterial reaction center. Nonetheless, they appear to be homologous the terminal 40% of each subunit is similar to a corresponding subunit of photosytem II. A special pair of chlorophyll a molecules lies at the center... 

All multi-subunit RNA polymerases have 5 core subunits. The (true) Bacteria have an additional a factor subunit that aids regulation and binding to DNA. 

Eukaryotic RNA polymerases have five core subunits plus five common subunits. 

Eukaryotes have three varieties of RNA polymerase. These differ slightly from one another RNA polymerase II has die five core subunits plus the five common subunits, but also has two variable subunits specific to RNA polymerase II only. Note that RNA polymerase II has a tail, which is involved in RNA processing and the initiation of transcription (Fig. 11.7). 

All Three have the core Subunits aian are 3,11 in eukaryotes (heterodimer), become 2,1 and ca s equivalent is 6. AU three also have five common extra subunits 5, 8, 9,10 and 12, and a variable number of specific subunits. 

The intracellular introduction of synthetic molecules that bind to a specific component (or components) of the holoenzyme telomerase can serve to literally switch off the activity of telomerase once it is already in full-swing (as is the case in most cancer cells). To date, the agents employed to successfully down-regulate or inhibit telomerase activity by directly binding to one of its components have mainly targeted hTERT and hTR, the core subunits of telomerase. These inhibition agents are shown in Figures 3A and 3B. 

Type II PKS complexes are comprised at a minimum of four types of subunits encoded by discrete open reading frames acyl carrier protein, ketosynthase a, ketosynthase p (also referred to as chain length factor ), and a malonyl CoA acyltransferase responsible for loading acyl-CoA extender units on to the acyl carrier protein subunit (34 Fig. 4). Additional subunits containing ketoreductase, cyclase, or aromatase activity may also occur in more complex type II synthases. Typically, the four core subunits (acyl carrier protein, ketosynthase a, ketosynthase p, and malonyl-CoA acyltransferase) participate in the iterative series of condensation reactions until a specified polyketide chain length is achieved, then folding and cyclization reactions yielding the final... 

A FIGURE 11-6 Schematic representation of the subunit structure of the E. coli RNA core polymerase and yeast nuclear RNA polymerases. All three yeast polymerases have five core subunits homologous to the p, p, two a. and co subunits of E. coli RNA polymerase. The largest subunit (RPB1) of RNA polymerase II also contains an essential C-terminal domain (CTD). RNA polymerases I and III contain the same two nonidentical a-like subunits, whereas RNA polymerase II contains two other nonidentical a-like subunits. All three polymerases share the same co-like subunit and four other common subunits. 

Figure 11-6). Each of the eukaryotic polymerases also contains an (O-lIke and two nonIdentIcal ct-llke subunits. The extensive similarity In the structures of these core subunits In RNA polymerases from various sources Indicates that this enzyme arose early In evolution and was largely conserved. This seems logical for an enzyme catalyzing a process so basic as copying RNA from DNA. 

In addition to their core subunits related to the E. coli RNA polymerase subunits, all three yeast RNA polymerases contain four additional small subunits, common to them but not to the bacterial RNA polymerase. Finally, each eukaryotic polymerase has several enzyme-specific subunits that are not present In the other two polymerases. Gene-knockout experiments In yeast Indicate that most of these subunits are essential for cell viability. Disruption of the few polymerase subunit genes that are not absolutely essential for viability nevertheless results in very poorly growing cells. Thus It... 

Eukaryotes contain three types of nuclear RNA polymerases. All three contain two large and three smaller core subunits with homology to the (3, (3, a, and co subunits of E. coli RNA polymerase, as well several additional small subunits (see Figure 11-6). 

Cytochrome c oxidase is a highly conserved enzyme the three core subunits (I, II and III) encoded by the mitochondrial genome in eukaryotes has high amino acid sequence homology with prokaryotic and eukaryotic species. Detailed analysis of evolutionary conservation of lipid-binding sites in cytochrome c oxidase (Qin et al., 2007) shows that lipid binding sites are specific and... 

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