You have isolated two mutants of a normally pear-shaped microorganism that have lost their distinctive shape and are now round.

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 you call A and the other has a defect in a protein 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.Which of the following statements are consistent with your results (more than one answer may apply)?

(a) Protein A is an integral membrane protein that interacts with B, a peripheral membrane protein that is part of the cell cortex.
(b) Protein B is an integral membrane protein that interacts with A, a peripheral membrane protein that is part of the cell cortex.
(c) Proteins A and B are both integral membrane proteins.
(d) The mutation in A affects its ability to interact with B.

Answer: 

The endothelial cells found closest to the site of an infection express proteins called lectins. Each lectin binds to a particular ____________ that is presented on the surface of a target cell.

The endothelial cells found closest to the site of an infection express proteins called lectins. Each lectin binds to a particular ____________ that is presented on the surface of a target cell.

(a) oligosaccharide
(b) aminophospholipid
(c) polysaccharide
(d) sphingolipid

Answer: 

Both glycoproteins and proteoglycans contribute to the carbohydrate layer on the surface of the cell. Which of the following is not true of glycoproteins?

Both glycoproteins and proteoglycans contribute to the carbohydrate layer on the surface of the cell. Which of the following is not true of glycoproteins?

(a) They can be secreted into the extracellular environment.
(b) They have only one transmembrane domain.
(c) They have long carbohydrate chains.
(d) They are recognized by lectins.

Answer: 

Which of the following statements about the carbohydrate coating of the cell surface is false?

Which of the following statements about the carbohydrate coating of the cell surface is false?

(a) It is not usually found on the cytosolic side of the membrane.
(b) It can play a role in cell-cell adhesion.
(c) The arrangement of the oligosaccharide side chains is highly ordered, much like the peptide bonds of a polypeptide chain.
(d) Specific oligosaccharides can be involved in cell-cell recognition.

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