Subunit assembly

The structures of dehydrogenases provide an interesting example of the combination of different polypeptide sequences into a single chain 

McLaughlin and M. 0. Dayhoff, Atlas Protein Sequence Struct. 5, 47 (1972), 

In all dehydrogenases investigated so far there is an a-helix ( 3G in LDH and s-MDH) on the same side of the sheet as B and aC and running roughly parallel to these and the strands in the sheet. It is situated closest to /JD. However, in terms of sequence, this helix is invariably associated with the catalytic domain. Thus, in LDH it involves residues 249-263. In LADH (where it is called aA) it consists of residues 176-194, while in lobster GAPDH it forms the C-terminal helix with residues 31 -334. 

The structural differences of the catalytic domains are also reflected in differences in subunit interactions. For LDH, s-MDH, and GAPDH these have been expressed in terms of an orthogonal, right-handed coordinate system (P,Q,R) deflned by Rossmann et al. (57). In LDH they form the three mutually perpendicular molecular 2-fold axes. These axes can be defined with respect to the direction of the strands in the 3-pleated sheet. 

Appropriately the association across the catalytic domains can be more easily perturbed. In dimeric s-MDH, there is no further association owing 

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Kv 3 -subunits are auxiliary subunits of Shaker-related Kv-channels, which belong to the Kvl subfamily of voltage-gated potassium channels. Kv(3 -subunits may function as chaperones in Kva -subunit assembly and may modulate the gating properties of Kv-channels. In particular, some Kv(3 -subunits may confer a rapid inactivation to otherwise non-inactivating Kv-channels. 

In addition to the membrane-inserted core domain of Kv channels, their cytoplasmic domains have important roles for Kv-channel function [5]. Many of these functions are related to subunits assembly, channel trafficking to and from the plasma membrane, and interactions with cytoskeletal components (Fig. la). A tetramerization (T) domain for subunit assembly has been well defined in Shaker-channels, where it is localized in the amino-terminus. Other Kv-channels (e.g., eag, HERG, KvLQTl) may have comparable domains within the cytoplasmic carboxy-terminus. ER retention and retrieval signals have been found... 

The identification of inhibitors of virus subunit assembly has been an objective of virologists for several years but it is only recently that papers have been published that demonstrate the validity of this approach to antiviral chemotherapy. It is hoped that the information provided by the compounds described above will provide the foundation for the generation of potent antiviral drugs to combat diseases caused by HIV, HBV and other viruses. 

Ni, but it had the native enzyme oligomeric organization (46). Thus, the C-terminal extension is needed for Ni incorporation, which, in turn, is not needed for subunit assembly. 

The expansion of our knowledge of the structure and function of Na,K-ATPase is reflected in a rapid succession of reviews on Na,K-ATPase genes and regulation of expression [17], subunit assembly and functional maturation [20], the isozymes of Na,K-ATPase [18], and the stability of a subunit isoforms during evolution [21], physiological aspects and regulation of Na,K-ATPase [22], reconstitution and cation exchange [23], chemical modification [24], and occlusion of cations [25]. Other valuable sources are the review articles [26] and recent developments [27] reported at the International Na,K-pump Conference in September 1990. 

Igarashi, K., Fujita, N., and Ishihama, A. (1991). Identification ofa subunit assembly domain in the alpha subunit ofEscherichia coli RNA polymerase. J. Mol. Biol. 218, 1-6. 

Grinevich, V.P., Letchworth, S.R., Lindenberger, K.A. et al. Heterologous expression of human alpha 6 beta 4 beta 3 alpha 5 nicotinic acetylcholine receptors binding properties consistent with their natural expression require quaternary subunit assembly including the alpha 5 subunit. J. Pharmacol. Exp. Then 312 619, 2005. 

Fabian, J. R., Kimball, S. R., Heinzinger, N. K., and Jefferson, L. S. (1997). Subunit assembly and guanine nucleotide exchange activity of eukaryotic initiation factor-2B expressed in Sf9 cells. J. Biol. Chem. 272, 12359—12365. 

Proteasome jS-subunits are synthesized with N-terminal extensions and are inactive because a free N-terminal threonine is required for peptide-bond hydrolysis [130]. The precursor jS-subunits assemble with a-subunits to form half proteasomes com-... 

Eukaryotic ribosomal RNA is transcribed in the nucleolus by RNA polymerase I as a single piece of 45S RNA, which is subsequently deaved to yield 28S rRNA, 18S rRNA, and 5.8S rRNA. RNA polymerase III transcribes the 5S rRNA unit from a separate gene. TTie ribosomal subunits assemble in the nudeolus as the rRNA pieces combine with ribosomal proteins. Eukaryotic ribosomal subunits are 60S and 40S. They join during protein synthesis to form the whole SOS ribosome. 

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). 

The smaiier 40 S subunit consists of one moiecuie of 18 S rRNA and 33 protein moie-cuies. The iarger 60 S subunit contains three types of rRNA with sedimentation coef cients of 5 S, 5.8 S, and 28 S and 47 proteins. In the presence of mRNA, the subunits assemble to form the complete ribosome, with a mass about 650 times larger than that of a hemoglobin molecule. 

True self-assembly is observed in the formation of many oligomeric proteins. Indeed, Friedman and Beychok reviewed efforts to define the subunit assembly and reconstitution pathways in multisubunit proteins, and all of the several dozen examples cited in their review represent true self-assembly. Polymeric species are also formed by true self-assembly, and the G-actin to F-actin transition is an excellent example. By contrast, there are strong indications that ribosomal RNA species play a central role in specifying the pathway to and the structure of ribosome particles. And it is interesting to note that the assembly of the tobacco mosaic virus (TMV) appears to be a two-step hybrid mechanism the coat protein subunits first combine to form 34-subunit disks by true self-assembly from monomeric and trimeric com-... 

Subunit assembly in multisubunit proteins, BIOCHEMICAL SELF-ASSEMBLY Subzero enzymology,... 

Metman LV, DelDotto P, LePoole K, etal (1999) Amantadine for levodopa-induced dyskinesias—a 1-year follow-up study. Arch Neurol 56 1383-1386 Mirshahi T, Woodward JJ (1995) Ethanol sensitivity of heteromeric NMDA receptors effects of subunit assembly, glycine and NMDA receptorl mg2+-insensitive mutants. Neuropharmacology 34 347-355... 

Figure 6 shows the proposed subunit assembly structure of the nicotinic acetylcholine receptor channel." The inner wall of the lower half part is surrounded by hydroxyl side chains from Ser and Thr, and by carboxylates or amides from Asp, Glu, and Gin at the mouth. Furthermore, a Lys residue seems to offer ion pairing with the carboxylate at the mouth. Considering the possibly similar stabilizing effect of ether and hydroxyl groups to cations, the proposed artificial supramolecular channel could be regarded as a good model of the acetylcholine receptor channel, which selects cations over anions, but does not discriminate between alkali metals. 

Figure 6. Proposed inner wall structure of the nicotinic acetylcholine receptor-channel composite from a2pY8 subunit assembly. The channel mouth is constructed from charged amino acids and their amides such as Asp, Glu, and Gin. A Lys is located at just the inner mouth. The lower half is covered by the amino acids having hydroxyl such as Ser and Thr, while the upper half is lined up with hydrophobic residues such as Leu, Val, Ala, lie, and Phe.

Figure 7-20 (A) Subunit assembly of two C3 catalytic trimers (green) and three R2 regulatory dimers around the periphery in aspartate carbamoyltransferase. After Krause et a/.109 Courtesy of William N. Lipscomb. The aspartate-and carbamoylphosphate-binding domains of the catalytic subunits are labeled Asp and CP, respectivley, while the zinc and allosteric domains of the regulatory subunits are labeled Alio and Zn, respectively. (B) Ribbon drawing of a single pair of regulatory (left) and catalytic (right) subunits with the structural domains labeled. MolScript drawing from Thomas et al.no...

Specific applications of FRET and FRET include observation of myosin movement (Fig. 19-14),157 measurement of distances between binding sites on tubulin,161 determining stoichiometry of subunit assembly in a y-aminobutyrate receptor of brain,162 association of proteins in peroxisomes,160 study of hydridization of deoxyribonucleotides,163 verifying the handedness of various forms of DNA,164 and other studies of DNA and RNA.164a b... 

Scheme 3 Schreiber s subunit assembly and completion of discodermolide...

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