Proteins forming large multimeres

ITowever, membrane proteins can also be distributed in nonrandom ways across the surface of a membrane. This can occur for several reasons. Some proteins must interact intimately with certain other proteins, forming multisubunit complexes that perform specific functions in the membrane. A few integral membrane proteins are known to self-associate in the membrane, forming large multimeric clusters. Bacteriorhodopsin, a light-driven proton pump protein, forms such clusters, known as purple patches, in the membranes of Halobacterium halobium (Eigure 9.9). The bacteriorhodopsin protein in these purple patches forms highly ordered, two-dimensional crystals. 

All of the transport systems examined thus far are relatively large proteins. Several small molecule toxins produced by microorganisms facilitate ion transport across membranes. Due to their relative simplicity, these molecules, the lonophore antibiotics, represent paradigms of the mobile carrier and pore or charmel models for membrane transport. Mobile carriers are molecules that form complexes with particular ions and diffuse freely across a lipid membrane (Figure 10.38). Pores or channels, on the other hand, adopt a fixed orientation in a membrane, creating a hole that permits the transmembrane movement of ions. These pores or channels may be formed from monomeric or (more often) multimeric structures in the membrane. 

Some proteins are formed by a single chain and are called monomeric, but a large number are formed by several polypeptide chains that associate in a multimeric molecule. The relationships of the peptide chains in a multichain protein are known as the quaternary structure. These subunits may work either independently of each other or cooperatively, i.e. the function of one subunit depends on the functional state of the others11. 

There are over 2500 different biochemical reactions with specific proteinaceous enzymes adapted for their rate enhancement. Since different species of organism produce different structural variants of enzymes, the number of different enzyme proteins in all of biology is many millions. Each enzyme is characterized by specificity for a narrow range of chemically similar substrates (reactants) and also other molecules that modulate their activities these are called ejfectors and they can be activators, inhibitors, or both. In more complex enzymes, one compoimd may have either effect, depending on other physical or chemical conditions. Enzymes range in size from large multi-subimit complexes (called multimeric enzymes 10 ) to small single-subunit forms. 

Under appropriate conditions, isolated triskelions spontaneously reassociate in vitro to form a range of open and closed baskets of various sizes. In vivo assembly to a clathrin basket of the correct size and structure involves participation of additional proteins known as assembly proteins, e.g. monomeric clathrin assembly protein (APj, 180,000 different from the large triskelion polypeptide), monomeric auxilin M, 90,000), and a relatively small (M, 20,000) clathrin assembly protein that is similar or identical to myelin basic protein, as well as the multimeric adaptors. All of these proteins promote the in vitro assembly of triskelions into homogeneous populations of clathrin cages. In vivo, assembly proteins are thought to be located between the membrane of the C-c.v. and its clathrin coat. 

Triton X-100 extracts of Sendai virus and several strains of the closely related Newcastle disease virus (NDV) were subjected to SEC. The elution patterns are shown in Fig. 3. SDS-polyacrylamide gel electrophoresis (SDS-PAGE) showed that the tetramer and the dimer of the Sendai virus hemagglutinin-neuraminidase (HN) protein (272 and 136 kDa, respectively) and the monomeric fusion (F) protein (65 kDa) were present in peaks 1, 2, and 3, respectively. These peaks are followed by an extremely large peak that contains Triton X-100 micelles. The elution patterns of the other viruses show that there are notable differences when they are compared with that of Sendai virus. In some cases, for example, the strains La Sota and Texas, the most prominent protein eluting from the column is an HN monomer. In other cases, multimeric forms of the HN protein that differ from Sendai HN protein were eluted (strains Mukteswar, Florida, and Herts). 

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