You are interested in integral membrane proteins of the plas

You are interested in integral membrane proteins of the plasma membrane. You treatments western blotting 4 individual proteins isolated after the following CONTROL: From untreated cells SET 1: Intact cells treated with a protease enzyme. SET 2: cells made permeable to proteins by treatment with small amounts of detergent before treatment with the protease. Four different proteins (W, X, Y and Z) are analyzed on SDS-PAGE Why do the bands in the \"C\" lanes run with different mobilities? which protein is exposed only on the exterior of the cell? Explain. Which protein is exposed in the interior AND the exterior of the cell? Explain. What is the minimum size (in kD) of this protein? What can you say about the other two proteins in terms of topology? Markers in Kilodaltons C 1 2 C 1 2 C 1 2 C 1 2 62 50/48 37 15

Solution

A) Depending on their size, each protein moves differently through the gel matrix: small protein molecules more easily fit through the pores in the gel, while larger ones have more difficulty. After the fixed time, the protein molecules have migrated different distances based on their size. Smaller proteins travel farther distances i.e. down the gel, while larger ones travels smaller distances and remain closer to the point of origin. Proteins may therefore be separated roughly according to size, which depends mainly on molecular weight under denaturing conditions,

B) Extrinsic proteins also known as peripheral membrane proteins are attached only to the exterior surface of the memebrane. They do not interact with the hydrophobic portion of the phospholipid bilayer instead they interact directly or indirectly with the polar heads of the lipid bilayer and integral proteins. they generally adhere only temporarily. These help in the signal transduction.

C) Integral membrane proteins are present both inside and outside of the biological membrane. these are also known as transmembrane proteins. These are permanent type of proteins. These are either receptor proteins or from channels.

D) It is slightly more than 37 Kda and less than 48 Kda

E) The topology of a membrane proteins describes the segments of the polypeptide chain that form the transmembrane helices and their orientation relative to the membrane.