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.

\quad\quad\quad\quad\quad\quad\quad\text { First cell extract }

\quad\quad\quad\quad\quad\text { After urea wash and centrifugation }

\begin{array}{lcccc}\hline&\text { Membrane }&\text { Cytosol }&\quad\quad\text {Membrane }&\text {Supernatant }\\\hline\text { Normal cells } & \text { A and B } & \text { no A or B } & \text { A } & \text { B } \\\hline \text { Mutant A } & \text { A } & \text { B } & \text { A } & \text { no A or B } \\\hline \text { Mutant B } & \text { A } & \text { B } & \text { A } & \text { no A or B }\end{array}

Figure 11-15
Use these initial observations to classify membrane proteins A and B.What additional information can be inferred from this data set that helps you understand the mutant phenotype?

Which Mechanism Best Describes the Process by Which an Antigen-Presenting

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