Intrinsic proteins cell membrane

Two distinct classes of proteins are synthesized by membrane-bound polysomes in cells which have a well-defined rough endoplasmic reticulum. They are secreted proteins which are destined for export or for transport to other cellular organelles, and membrane intrinsic proteins which have to be inserted asymetrically toward the noncytoplasmic face of the membrane. The role of the RER for the synthesis of secretory proteins is well established (Palade, 1975). For these proteins, compartmentalization regulates the different processes. They must be transferred across the hydrophobic bilayers... 

AQPO, formerly known as the Major Intrinsic Protein of 26 kDa (MDP26), is specifically expressed in the plasma membrane of eye lens fiber cells. It transports water to a low degree, but has also been implicated in cell adhesion and gap junction formation. Its main role is to maintain the transparency of the lens by maintaining a tight cellular connection to neighboring cells and/or by controlling the fluid circulation. 

Gorin, M. B., Yancey, S. B., Cline, J., Revel, J. P. and Horwitz, J. (1984). The major intrinsic protein (MIP) of the bovine lens fiber membrane characterization and structure based on cDNA cloning, Cell, 39, 49-59. 

Much of industrial chemistry takes place in organic solvents, or involves apolar compounds. Biocatalysis, in contrast, typically involves aqueous environments. Nevertheless, enzymes and microorganisms do in fact encounter apolar environments in Nature. Every cell is surrounded by at least one cell membrane, and more complex eukaryotic cells contain large amounts of intracellular membrane systems. These membranes consist of lipid bilayers into which many proteins are inserted present estimates, based on genomic information, are that about one-third of all proteins are membrane proteins, many of which are so-called intrinsic proteins that are intimately threaded through the apolar bilayer. These proteins are essentially dissolved in, and function partly within, an apolar phase. 

Lipoproteins or glycoproteins are the macromolecules that most commonly form receptors. They are often firmly embedded in the plasma membrane or cell-organelle membrane as intrinsic proteins (see section 7.1). At times, this renders their isolation and subsequent functional reconstitution difficult, as their structure may be dependent upon the surrounding membrane. Isolation of such a receptor molecule may cause its structural collapse, even to the extent that specific binding properties are lost. 

Fig. 9.2. The GTPase cycle of the Ras protein. Conversion of the inactive Ras GDP complex into the active Ras GTP complex is brought about by guanine nucleotide exchange factors (GEFs). The activated state of the Ras protein is terminated by hydrolysis of the bound GTP. The help of a GTPase actvating protein (GAP) is required, due to the intrinsically slow GTPase activity of the Ras protein. Ras protein performs all its functions in close association with the cell membrane. It carries a membrane anchor and the effector proteins preceding and following in sequence are also associated with the membrane.

A few substances are so large or impermeant that they can enter cells only by endocytosis, the process by which the substance is bound at a cell-surface receptor, engulfed by the cell membrane, and carried into the cell by pinching off of the newly formed vesicle inside the membrane. The substance can then be released inside the cytosol by breakdown of the vesicle membrane. Figure 1-5D. This process is responsible for the transport of vitamin B12, complexed with a binding protein (intrinsic factor) across the wall of the gut into the blood. Similarly, iron is transported into hemoglobin-synthesizing red blood cell precursors in association with the protein transferrin. Specific receptors for the transport proteins must be present for this process to work. 

Thus, the fat globules are surrounded, at least initially, by a membrane typical of eukaryotic cells. Membranes are a conspicuous feature of all cells and may represent 80% of the dry weight of some cells. They serve as barriers separating aqueous compartments with different solute composition and as the structural base on which many enzymes and transport systems are located. Although there is considerable variation, the typical composition of membranes is about 40% lipid and 60% protein. The lipids are mostly polar (nearly all the polar lipids in cells are located in the membranes), principally phospholipids and cholesterol in varying proportions. Membranes contain several proteins, perhaps up to 100 in complex membranes. Some of the proteins, referred to as extrinsic or peripheral, are loosely attached to the membrane surface and are easily removed by mild extraction procedures. The intrinsic or integral proteins, about 70% of the total protein, are tightly bound to the lipid portion and are removed only by severe treatment, e.g. by SDS or urea. 

Fc receptors (FcR) are a family of detergent-soluble membrane glycoproteins with approximate molecular weights of 50-70 kD. They comprise less than one percent of the total membrane proteins and are most frequently present on macrophages and granulocytes. They have also been reported on B cells and some T cells. The intrinsic affinity of the FcR for monomeric IgG is approximately 1x106 to 1x108... 

Hereditary spherocytosis is a group of disorders caused by heterogeneous intrinsic defects of the red cell membrane proteins. Describe the rapid and sensitive methods to detect mutations in genomic DNA and complementary DNA. 

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