Membranes proteins, classes

Membrane proteins comprise another important class of protein crystallized in 2D. These proteins perform important functions as membrane channels and recognition sites for cells. Unlike the streptavidin crystals, membrane proteins... 

Singer and Nicolson also pointed out that proteins can be associated with the surface of this bilayer or embedded in the bilayer to varying degrees (Figure 9.6). They defined two classes of membrane proteins. The first, called peripheral proteins (or extrinsic proteins), includes those that do not penetrate the bilayer to any significant degree and are associated with the membrane by virtue... 

K Channels belong to a class of membrane proteins that form highly K-selective pores in membranes. All known K Channels are composed of several (usually four) pore forming alpha subunits and auxiliary beta subunits. K Channels play an essential role in cellular excitability, being involved in repolarization of Action Potentials and setting the cell resting potential as well as contributing to potassium homeostasis. 

Scientists initially approached structure-function relationships in proteins by separating them into classes based upon properties such as solubility, shape, or the presence of nonprotein groups. For example, the proteins that can be extracted from cells using solutions at physiologic pH and ionic strength are classified as soluble. Extraction of integral membrane proteins requires dissolution of the membrane with detergents. 

Class 1 MHC molecules are integral membrane proteins found on the surface of all nucleated cells and platelets. They are the classical antigens involved in graft rejection. 

G-Protein coupled receptors (GPCR) represent the start element in secondary messenger producing systems. They comprise a family of over 1000 structurally-related members. These membrane proteins are also called serpentine or seven-helix receptors due to their seven transmembrane domains with an a-helical conformation. Receptors belonging to this class respond to a variety of hormones and neurotransmitters, and they detect odorant molecules or light [3,4]. 

Banerjee R, Jao JB, Bush JT, Dawson G. Differential solubilization of lipids along with membrane proteins by different classes of detergents. Chem Phys Lipids 1995 77 65-78. 

Peptides may bind to a membrane either by association to its surface or by insertion into its interior. The latter class comprises the integral membrane proteins whose structures are largely a-helical or / -barrel type (see Chap. 12 in Ref. [32]). The topology of interaction of helices with membranes is displayed in Fig. 5.1. 

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