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Integral Proteins Are Of Two Types: Transmembrane Proteins And Lipid Anchored Proteins

914 words - 4 pages

Integral proteins are the main type of proteins in membrane proteins. They not only transport and receive nutrients and ions; they also carry out most membrane functions (Alberts et al. 2004). In integral proteins, there are usually two main types of proteins, transmembrane proteins and lipid anchored proteins. The former ones extend through the lipid bilayer. A hydrophobic domain of the protein lies in the interior of the bilayer and their hydrophilic regions are exposed to the outside on either side of the membrane (Medvedeva 2003). The latter ones are located entirely outside the bilayer, and are connected to the bilayer with one or more covalently attached lipid groups (Wikipedia 2005).In-Gel Trypsin digestion experiment plays an important role in determining the two different types of integral proteins. Trypsin is a special digestive enzyme that can degrade proteins to amino acids (Aman and Wang 1987). Trypsin enzyme is a very big molecule and it is too large to go through the membrane (Aman and Wang 1987). As a result, when the membrane is treated with trypsin, the proteins, which locate on the outside of the lipid bilayer, are digested by trypsin. However, the proteins that are located in the interior of the bilayer are not degraded. After the membranes been treated with trypsin, the digested membranes will be isolated from the membranes with detergent and separate by electrophoresis (Medvedeva 2003). Electrophoresis is the main technique for separating molecules. For protein extraction, the detergent SDS (sodium dodecyl sulfate) is used as the support medium, and all proteins on membranes become negatively charged by their attachment to the SDS anions (Medvedeva 2003). Usually the proteins will be separated on a polyacrylamide gel, and the whole procedure is called SDS-PAGE (Sodium Dodecyl Sulfate PolyAcrylamide Gel Electrophoresis) (Medvedeva 2003). At the beginning, the proteins stay on the top of the gel, but under the influence of an electrical field, they will move downward along the gel (Aman and Wang 1987). The smaller proteins move faster than the bigger ones, so the proteins at the top of the gel are the largest and those at the bottom are the smallest (Aman and Wang 1987). This will also help us determine if there are two types of integral proteins inside the membrane. If they do have transmembrane proteins, some smaller proteins will be appeared at the bottom of the gel after the membrane being treated with trypsin.In 1987 the experiment done by Boyd, Manoil, and Beckwith showed us the topology of the integral membrane protein. Even though, the main purpose of this experiment was to identify the structure and activity of integral proteins in the membranes, we can still determine two different types of integral proteins through part of the experiment. In this experiment, the integral membrane protein MaIF, which is required for maltose...

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