Domain and subunit assembly

The original postulate of a linear model for basic unit/T-domain and subunit assembly in solution was based on light-scattering studies on the... 

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... 

FIGURE 12-3 lonophore receptors for neurotransmitters are composed of subunits with four transmembrane domains and are assembled as tetramers or pentamers (at right). The predicted motif shown likely describes nicotinic cholinergic receptors for ACh, GABA receptors for gamma-aminobutyric acid, and receptors for glycine. 

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. 

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. 

There are at least two assembly domains, namely the L20 domain and the L15 domain, in the 50 S assembly map (Fig. 15). Proteins within the L20 domain are essential for the assembly but not for the function of the 50 S subunit whereas those in the L15 domain are functionally important proteins whose assembly occurs at a late state. As with the 30 S subunit, the assembly map of the 50 S subunit (Rohl and Nierhaus, 1982) not only reflects the assembly dependence but also the topographical relationship of the proteins within the ribosomal particle. This conclusion is supported by a good correspondence between the assembly map on the one hand, and results from cross-linking studies and from the sequential removal of proteins from the particle by LiCl on the other hand. There is also a correlation between the interdependence of proteins during the assembly process and the arrangement of their genes on the E. coli chromosome (Rbhl et al., 1982). 

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...

Fig. 6-24 Schematic representation of the epidermal growth factor (EGF) receptor. The receptor is an integral membrane protein with a single transmembrane domain. The ligand binding site is in the extracellular domain and there is a tyrosine kinase domain near the C terminus in the cytoplasm, (a) At rest the receptor exists as single subunits. (b) Upon binding EGF, the receptor forms dimers stabilized by noncovalent associations. After dimerization the activated tyrosine kinase phosphorylates tyrosine residues in the cytoplasmic domain prior to the recruitment of further proteins to bind to the receptor. The formation of a protein assembly on the cytoplasmic domain is necessary for activation of enzymes that regulate cell metabolism and gene transcription.

The crystal structure of Tb MscL established that this protein assembles as a homopentamer that is organized into two domains, the transmembrane domain and the cytoplasmic domain. The transmembrane domain consists of 10 helices (2 per subunit) connected by an extracellular loop, while the cytoplasmic domain contains 5 helices that form a left-handed pentameric bundle. The sequence of the Tb.MscL subunit has 151 amino acids and can be further subdivided into five segments the N terminus, the first transmembrane helix (TM1), an extracellular loop, the second transmembrane helix (TM2), and a cytoplasmic domain (Fig. 4, see color insert). Each of the segments is discussed in more detail below. The pore is aligned along the fivefold symmetry axis and is formed by the first transmembrane helix (TM1) and an extracellular loop from each subunit. The channel has overall dimensions of approximately 85 x 50 x 50 A and both the N and C termini reside on the... 

The reconstitution of bacterial ribosomal subunits from the separated rRNAs and proteins, first announced in 1968, provides a potent tool to investigate such essential aspects of ribosome structure and evolution as the subunits assembly pathway [92,93], the locations and neighbourhoods of the subunit proteins [94,95], the roles of the individual proteins in both assembly and function [92,93,96], and the degree of exchangeability of ribosomal components both within and across domain boundaries [97,98]. 

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