Bacteria 3-type membrane protein structure

Reaction centers of purple bacteria. The exact composition varies, but the properties of reaction centers from several genera of purple bacteria are similar. In Rhodopseudomonas viridis there are three peptide chains designated H, M, and L (for heavy, medium and light) with molecular masses of 33,28, and 24 kDa, respectively. Together with a 38-kDa tetraheme cytochrome (which is absent from isolated reaction centers of other species) they form a 1 1 1 1 complex. This constitutes reaction center P870. The three-dimensional structure of this entire complex has been determined to 0.23-nm resolution288 319 323 (Fig. 23-31). In addition to the 1182 amino acid residues there are four molecules of bacteriochlorophyll (BChl), two of bacteriopheophytin (BPh), a molecule of menaquinone-9, an atom of nonheme iron, and four molecules of heme in the c type cytochrome. In 1984, when the structure was determined by Deisenhofer and Michel, this was the largest and most complex object whose atomic structure had been described. It was also one of the first known structures for a membrane protein. The accomplishment spurred an enormous rush of new photosynthesis research, only a tiny fraction of which can be mentioned here. 

Diversity and heterogeneity of the MS class of channels is weh illustrated by the fact that MscS, the second type of MS channels fonnd in bacteria, differs signihcantly from MscL in its primary as well as qnaternary strncture. Each channel belongs to a separate snbfamily of the large family of prokaryotic MS channels (3). A monomer of MscS is a small membrane protein of 286 amino acids. A 3-D crystal structure of MscS shows that the functional channel is a homoheptamer having a large, cytoplasmic region (Fig. lb) (8). Each of the... 

Porin, a protein from the outer membranes of bacteria such as E. coli and Rhodo-bacter capsulatus, represents a class of membrane proteins with a completely different type of structure. Structures of this type are built from P strands and contain essentially no a helices (Figure 12.20). 

The type III secretion apparatus, used by pathogenic bacteria to inject proteins into eukaryotic cells, consists of a basal portion that spans both membranes and an extracellular needlelike structure that can penetrate the plasma membrane of a target cell (see Figure 16-24). 

Let s begin on a small scale the architecture of cells. The cells of plants and bacteria have strong cell walls that provide for maintenance of shape and protection against ontside forces. The cells of animals, including those of the human body, in contrast, lack cell walls. Animal cells make do with a fragile cell membrane. Our cells, consequently, have need of an internal architecture to meet the needs supplied by cell walls in other life forms. Cells have three types of internal architectural elements microtubules, intermediate filaments, and actin filaments. Each of these structures is composed of protein. 

Other bacterial coats. Archaebacteria not only have unusual plasma membranes that contain phytanyl and diphytanyl groups (Section A,3)608 but also have special surface layers (S-Iayers) that may consist of many copies of a single protein that is anchored in the cell membrane.609 The surface protein of the hypothermic Staphylothermus marius consists of a complex structure formed from a tetramer of 92-kDa rods with an equal number of 85-kDa "arms."610 611 S-layers are often formed not only by archaebacteria but also by eubacteria of several types and with quite varied structures.612 14 While many bacteria carry adhesins on pili, in others these adhesive proteins are also components of surface layers.615 Additional sheaths, capsules, or slime layers, often composed of dextrans (Chapter 4) and other carbohydrates, surround some bacteria. 

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