Bacteriorhodopsin barrel proteins

Membranes contain many largely a-helical proteins. Cell surface receptors often appear to have one, two, or several membrane-spanning helices (see Chapter 8). The single peptide chain of the bacterial light-operated ion pump bacteriorhodopsin (Fig. 23-45) folds back upon itself to form seven helical rods just long enough to span the bacterial membrane in which it functions.189 Photosynthetic reaction centers contain an a helix bundle which is formed from two different protein subunits (Fig. 23-31).190 A recently discovered a,a barrel contains 12 helices. Six parallel helices form an inner barrel and 6 helices antiparallel to the first 6 form an outer layer (see Fig. 2-29).191-193... 

Figure 4 Two examples of membrane proteins, (a) Bacteriorhodopsin is mainly an a-protein containing seven helices. It is a membrane protein serving as an ion pump and is found in bacteria that can survive in high salt concentrations, (b) Porin is a P-barrel. Porins work as channels in cell membranes, which let small metabolites such as ions and amino acids in and out of the cell. Figure drawn using MOLSCRIPT. ...

---