Question

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11. (20 points) You have isolated two mutants of a normally pear-shaped microorganism that have lost their distinctive shape and are now round. One of the mutants has a defect in a protein that you call A and the other has a defect in a protein that you call B. First, you grind up each type of mutant cell and normal cells separately and separate the plasma membranes from the cytoplasm, forming the first cell extract. Then you set aside a portion of each fraction for later testing. Next, you wash the remaining portion of the membrane fractions with a low concentration of urea (which will unfold proteins and disrupt their ability to interact with other proteins) and centrifuge the mixture. The membranes and their constituent proteins form a pellet, and the proteins liberated from the membranes by the urea wash remain in the supernatant. When you check each of the fractions for the presence of A or B, you obtain the results given below. After urea wash and centrifugation Membrane Supernatant First cell extract Membrane cytosol A and B no A or B AB A Normal cells Mutant A Mutant B А A no A or B no A or B A Use these observations to classify the type of membrane proteins A and B. What additional information can be inferred from this data set that helps you understand the mutant phenotype?

Answer 1

From the first cell extract we infer that both protein A and B are membrane proteins since they are present in the menbrane of the normal cells.

Also,

Membrane proteins are of two types peripheral and intergral proteins.

Peripheral proteins are present on the inner or outer surface of the membrane and integral proteins are present buried deep inside the membrane.Peripheral proteins asy come out dissociate during the process of centrifugation whereas integral proteins do not and stay within the membrane .

And after treatment with urea if is found that protein B is present in the supernatant .

So from the factors mentioned and the observation Protein A can be identified as an integral membrane protein and Protein B as a peripheral membrane protein.

As in the mutants simce these proteins are altered results in the altered membrane integrity and thereby has lead to their shape change once again proving that A and B are membrane proteins and has a definite role in the maintenance of the shape of the cell.