Subunit association

Harrison, S.C. Multiple modes of subunit association in the structures of simple spherical viruses. Trends Biochem. Sci. 9 345-351, 1984. 

If the protein of interest is a heteromultimer (composed of more than one type of polypeptide chain), then the protein must be dissociated and its component polypeptide subunits must be separated from one another and sequenced individually. Subunit associations in multimeric proteins are typically maintained solely by noncovalent forces, and therefore most multimeric proteins can usually be dissociated by exposure to pEI extremes, 8 M urea, 6 M guanidinium hydrochloride, or high salt concentrations. (All of these treatments disrupt polar interactions such as hydrogen bonds both within the protein molecule and between the protein and the aqueous solvent.) Once dissociated, the individual polypeptides can be isolated from one another on the basis of differences in size and/or charge. Occasionally, heteromultimers are linked together by interchain S—S bridges. In such instances, these cross-links must be cleaved prior to dissociation and isolation of the individual chains. The methods described under step 2 are applicable for this purpose. 

The forces that stabilize quaternary structure have been evaluated for a few proteins. Typical dissociation constants for simple two-subunit associations... 

One general benefit of subunit association is a favorable reduction of the protein s surface-to-volume ratio. The surface-to-volume ratio becomes smaller as the radius of any particle or object becomes larger. (This is because surface area is a function of the radius squared and volume is a function of the radius cubed.) Because interactions within the protein usually tend to stabilize the protein energetically and because the interaction of the protein surface with... 

The pore-forming oq subunit alone is sufficient to form a functional calcium channel in the cell membrane [8]. However, high-voltage-activated calcium channels are hetero-oligomeric structures that may contain as many as three additional smaller subunits, labeled CX25 (itself a heterodimer), (5, and y. These three subunits associate in an auxiliary... 

The sodium and calcium pumps can be isolated to near purity and still exhibit most of the biochemical properties of the native pump. Some kinetic properties of these pumps in native membranes are altered or disappear as membrane preparations are purified. For example, when measured in intact membranes, the time-dependencies of phosphorylation and dephosphorylation of the pump catalytic sites exhibit biphasic fast to slow rate transition this characteristic progressively disappears as the membranes are treated with mild detergents. One suggested explanation is that, as the pumps begin to cycle, the catalytic subunits associate into higher oligomers that may permit more efficient transfer of the energy from ATP into the ion transport process [29, 30], Some structural evidence indicates that Na,K pumps exist in cell membranes as multimers of (a 3)2 [31]. 

Tor several of the effector proteins listed, the functional regulation of the effector is mediated, in part or in full, by the [ly subunits associated with the indicated a subunit. 

Trudell, J. (2002) Unique assignment of inter-subunit association in GABA(A) alpha 1 beta 3 gamma 2 receptors determined by molecular modeling. Biochim. Biophys. Acta 1565,91-96. 

Peterson, S. R., D. M. Gadbois, E. M. Bradbury, and P. M. Kraemer. 1995. Immortalization of human fibroblasts by SV40 large T antigen results in the reduction of cyclin D1 expression and subunit association with proliferating cell nuclear antigen and Wafl. Cancer Res 55(20) 4651-7. 

Cholinesterases are subdivided into acetylcholinesterase and cholinesterase, one with a narrow, the other with broad substrate specificity [109-112], Both enzymes exist in multiple molecular forms distinguishable by their subunits association (Fig. 2.4). The hydrodynamic properties of these associations have allowed globular (G) and asymmetric (A) forms to be distinguished. The G forms can be hydrophilic (water-soluble, and excreted into body fluids) or amphiphilic (membrane-bound). The homomeric class exists... 

Subunit contacts need to be relatively extensive and stable if they are to ensure subunit association in the absence of a covalent link. However, in some cases a subunit contact can shift back and forth between two different stable positions, as has been demonstrated for oxy- versus deoxyhemoglobin (Perutz, 1970). Allosteric control can then be exerted by any factors which either affect the local conformation or bind between the subunits. A less elegant but even more extreme example is lamprey hemoglobin, which dissociates altogether in the oxy form (Hendrickson and Love, 1971). 

Karniol, B., Yahalom, A., Kwok, S., Tsuge, T., Matsui, M., Deng, X. W., Chamovitz, D. a. The Arabidopsis homologue of an el F3 complex subunit associates with the COP9 complex. FEBS Lett. 1998, 439, 173-179. 

Phillips HA, Favre I, Kilpatrick M, Zuberi SM, Goudie D, Heron SE, Scheffer IE, Sutherland GR, Berkovic SF, Bertrand D, Mulley JC (2001) CHRNB2 is the second acetylcholine receptor subunit associated with autosomal dominant nocturnal frontal lobe epilepsy. Am J Hum Genet... 

Fig. 12. A hypothetical folding and assembly pathway for catalases. In A secondary and tertiary folding first occurs in the individual subunits to form the 3-barrel (p), wrapping domain (W), a-helical segment (a), and fiavodoxin domain (F, only in HPII). In proceeding to B, heme is bound to each of the subunits, and this may serve as a catalyst for the rapid association of the i -related subunits to form the structure in C. In proceeding to D, Q-related subunits associate, resulting in the N-terminal arms being overlapped as the C-terminal portions fold back on themselves to form the fully folded structure shown in E. Only two subunits are shown in the progression from C to E, but a simultaneous folding must be occurring in the associated dimer. The fully folded tetramer is shown in two orientations.

Signal transduction at G-protein-cou-pled receptors uses essentially the same basic mechanisms (A). Agonist binding to the receptor leads to a change in receptor protein conformatioa This change propagates to the G-protein the a-subunit exchanges GDP for GTP, then dissociates from the two other subunits, associates with an effector protein, and alters its functional state. The a-subunit slowly hydrolyzes bound GTP to GDP. 

DNA polymerase III is much more complex than DNA polymerase I, having ten types of subunits (Table 25-2). Its polymerization and proofreading activities reside in its a and e (epsilon) subunits, respectively. The 6 subunit associates with a and e to form a core polymerase, which can polymerize DNA but with limited processivity. Two core polymerases can be linked by... 

DNA polymerase III can polymerize DNA, but with a much lower processivity than one would expect for the organized replication of an entire chromosome. The necessary increase in processivity is provided by the addition of the J8 subunits, four of which complete the DNA polymerase III holoenzyme. The J3 subunits associate in pairs to form donut-shaped structures that encircle the DNA and act like clamps (Fig. 25-10b). Each dimer associates with a core subassembly of polymerase III (one dimeric clamp per core subassembly) and slides along the DNA as replication proceeds. The J8 sliding clamp prevents the dissociation of DNA polymerase III from DNA, dramatically increasing processivity—to greater than 500,000 (Table 25-1). 

Quaternary structure of isoenzymes Many isoenzymes contain different subunits in various combinations. For example, creatine kinase occurs as three isoenzymes. Each isoenzyme is a dimer composed of two polypeptides (called B and M subunits) associated in one of three combinations CK1 = BB, CK2 = MB, and CK3 - MM. Each CK isoenzyme shows a characteristic electrophoretic mobility (see Figure 5.21). 

Transhydrogenases function in a similar way within bacteria. Whether from E. coli, photosynthetic bacteria, or bovine mitochondria, transhydrogenases have similar structures.285 Two 510-residue a subunits associate with two 462-residue P subunits to form an OC2P2 tetramer with 10-14 predicted transmembrane helices. Tire a subunits contain separate NAD(H) and NADP(H) binding sites. A conformational change appears to be associated with the binding or release of the NADP+ or NADPH 287... 

A (3 replacement reaction catalyzed by the PLP-dependent tryptophan synthase converts indoleglycerol phosphate and serine to tryptophan. Tryptophan synthase from E. coli consists of two subunits associated as an a2P2 tetramer (Fig. 25-3). The a subunit catalyzes the cleavage (essentially a reverse aldol) of indoleglycerol phosphate to glyceraldehyde 3-phosphate and free indole (Fig. 25-2, step s).67 The P subunit contains PLP. It presumably generates, from serine, the Schiff base of aminoacrylate, as indicated in Fig. 25-2 (step f). The enzyme catalyzes the addition of the free indole to the Schiff base to form tryptophan. The indole must diffuse for a distance of 2.5 ran... 

The mode of substrate binding may be monohapto or polyhapto (t ), depending on whether fixation occurs via a single or via multiple (substrate group-binding subunit) association(s) (see also [2.11]). 

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