Subunits, protein

We have seen in the structure of this simple satellite virus that 60 subunits are sufficient to form a shell around an RNA molecule that codes for the subunit protein, but there is little room for additional genetic information. 

The ion channel receptors are multi-subunit proteins which may be either homomeric (made up of multiple copies of a single type of subunit) or heteromeric (composed of more than one subunit type). These subunits come together after synthesis in the endoplasmic reticulum to form the mature receptor. Notice that stoichiometry is denoted by a subscript number. A receptor composed of two a and three /I subunits is therefore denoted as having a stoichiometry of This can cause confusion when related subunits are given sequential numbers /II, j]2, 3, etc. The convention is therefore that subunits are numbered normally while stoichiometry is indicated by subscripts so that a pentamer of a4 and j33 subunits might have a stoichiometry of a42/133. 

Figure 1. SDS-PAGE and Immunoblot Analysis of Purified PGl and Separated PG2 and Subunit Proteins.

Figure 10. Tissue specific expression of p-subunit proteins in floral tissues. Stage 3 (fully opened) flowers were collected, dissected and cell wall proteins (5 pgm) from the indicated organs isolated and analyzed for p-subunit antigen. Note the high level of expression in stigma/style and anthers/pollen and restriction of the larger antigen to stigma/style tissues. PGl lane, 1 gg of purified fruit PGl protein.

Larger proteins often contain more than one polypeptide chain. These multi-subunit proteins have a more complex shape, but are still formed from the same forces that twist and fold... 

Figure 21.2 Conceptualized construction of an A-B subunit protein toxin (left). The B chain contains a binding region for docking onto cell surfaces, while the A chain contains a catalytic site that produces cytotoxic affects intracellularly. The two subunits are joined by a disulfide bond that is reductively cleaved at the cellular level to allow the A subunit to affect cell death. A molecular model of ricin is on the right.

A-chain immunotoxins, however, may not be quite as cytotoxic as conjugates formed from intact toxin molecules (Manske et al., 1989). In an alternative approach to A chain use, the intact toxin of two-subunit proteins is directly conjugated to a monoclonal without isolation of the A chain. Conjugation of an antibody with intact A-B chain toxins can be done without a cleavable linker, as long as the A chain can still separate from the B chain once it is internalized. Therefore, it is important to avoid intramolecular crosslinking during the conjugation process which can prevent release of the A-B complex. In addition, since the B chain... 

Two of the most widely spread and well-studied enterotoxigenic forms of bacterial diarrhea are ETEC and Vibrio cholerae. The toxins they produce, labile toxin (LT) and cholera toxin (CT) respectively, are very similar in primary sequence, structure, and mechanism of action [72]. They are homologous multi-subunit proteins in which the non-toxic B subunit mediates GMj ganglioside binding, and thus are candidates for vaccines that can neutralize toxin activity. 

Monomeric TNF is biologically inactive the active form is a homotrimer in which the three monomers associate non-covalently about a threefold axis of symmetry, forming a compact bellshaped structure. X-ray crystallographic studies reveal that each monomer is elongated and characterized by a large content of antiparallel P pleated sheet, which closely resembles subunit proteins of many viral caspids (Figure 9.4). 

Clathrin-coated vesicles mediate transport from the Golgi apparatus to endosomes, and from the plasma membrane to endosomes. A multi-subunit protein, clath-rin, constitutes the major protein of this vesicle type (see Ch. 2). Clathrin is composed of three large and three small polypeptide chains, which assemble to form a triskelion (Fig. 9-2). Regulatory mechanisms control the assembly and formation of a convex, polyhexa-pentagonal basketlike structure by these triskelions [5], This structure is responsible for the formation of coated pits on the cytosolic face of plasma membranes. 

Knockout mice have also revealed further subunit requirements for the assembly of particular GABAa receptor subtypes. Thus, the absence of the a6 subunit in cerebellar granule cells results in a complete loss of 5 subunit protein, although mRNA expression levels were unaffected (59,62). These results indicate that the 8 subunit cannot be incorporated in a receptor lacking a() in cerebellar granule cells. Likewise, in 50/0 mice, the levels of a4 subunit protein are reduced in all forebrain regions in which the 8 subunit is normally abundant, pointing to a preferential co-assembly of these subunits in several neuronal populations (63). 

About a quarter of the total body iron is stored in macrophages and hepatocytes as a reserve, which can be readily mobilized for red blood cell formation (erythropoiesis). This storage iron is mostly in the form of ferritin, like bacterioferritin a 24-subunit protein in the form of a spherical protein shell enclosing a cavity within which up to 4500 atoms of iron can be stored, essentially as the mineral ferrihydrite. Despite the water insolubility of ferrihydrite, it is kept in a solution within the protein shell, such that one can easily prepare mammalian ferritin solutions that contain 1 M ferric iron (i.e. 56 mg/ml). Mammalian ferritins, unlike most bacterial and plant ferritins, have the particularity that they are heteropolymers, made up of two subunit types, H and L. Whereas H-subunits have a ferroxidase activity, catalysing the oxidation of two Fe2+ atoms to Fe3+, L-subunits appear to be involved in the nucleation of the mineral iron core once this has formed an initial critical mass, further iron oxidation and deposition in the biomineral takes place on the surface of the ferrihydrite crystallite itself (see a further discussion in Chapter 19). 

As described in Section II,B,1, this pathway seems to allow folded and oligomerized proteins to be translocated across the membrane. Therefore, it is not always necessary for a subunit protein to have the signal if another has one (so-called hitchhiker or piggy-backing mechanism) (Rodrigue etal., 1999). In this case, the prediction of its localization site would be inherently difficult. 

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