Immuno. Chapter 3 BCR/TCR study questions

Under activated conditions, where is N FAT localized in a cell?

Nucleus

Describe how the following experimental manipulations were used to determine antibody structure. Enzymatic digestion of the antibody molecule

Papain digested the molecule in the hinge region, releasing two Fabs and one Fc. This told investigators that there were two antigen-binding sites per molecule and suggested that part of the molecule was not variable in sequence.

True or false? Explain your answers. Interactions between receptors and ligands at the cell surface: are mediated by covalent interactions.

False. Receptors and ligands bind through noncovalent interactions such as hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals interactions.

Define an adapter protein. Describe how an interaction between proteins bearing SH2 and phosphorylated tyrosine p Y groups helps to transduce a signal from the T-cell receptor to the ZAP-70 protein kinase.

An adapter protein bears more than one binding site for other proteins and brings other proteins into contact without, itself, having any enzymatic activity. Activation of the TCR results in activation of the Src-family kinase, Lck.

Describe how the following experimental manipulations were used to determine antibody structure. Antibody detection of immunoglobulin fragments

Antibodies were generated that specifically recognized the Fab and Fc fragments. Antibodies to Fab were found to bind to both h and l chains, thus indicating that the antigen- binding sites had components of both chains. Antibodies to Fc fragments bound only to the heavy chain.

The B- and T-cell receptor proteins have remarkably short intracytoplasmic regions of just a few amino acids. How can you reconcile this structural feature with the need to signal the presence of bound antigen to the interior of the cell?

Both the BCR and the TCR are noncovalently associated on their respective cell membranes with signal transduction complexes that function to transduce the signal initiated by antigen binding to the receptor into the interior of the cell.

How is N FAT released from its resting condition and permitted to relocalize?

By de-phosphorylation with the enzyme calcineurin phosphatase, activated by binding to the calcium-calmodulin complex. Activation of the cell results in an increase in intracytoplasmic calcium ion concentration.

Under resting conditions, where is N FAT localized in a cell?

Cytoplasm

I g M has ten antigen-binding sites per molecule, whereas I g G only has two. Would you expect I g M to be able to bind five times as many antigenic sites on a multivalent antigen as I g G? Why or why not?

I would expect I g M to be able to bind more molecules of antigen than I g G, but perhaps not five times more. Steric hindrance/conformational constraints may prevent all 10 antigen-binding sites of I g M from being able to simultaneously bind to 10 antigenic sites.

In the early days of experiments designed to detect the T-cell receptor, research groups found that antibodies directed against I G proteins appeared to bind to the T-cell receptor. Given what you know about the structure of immunoglobulins and the T-cell receptor, why is this not completely surprising?

Immunoglobulin proteins and the T-cell receptor share a common motif: the immunoglobulin fold, which may be the binding target for these antibodies.

What is an ITAM, and what proteins modify the ITAMs in I g alpha and I g beta?

Immunoreceptor Tyrosine-based Activation Motif. It is a motif with a particular sequence containing phosphorylatable tyrosines in the cytoplasmic regions of several immunologically important proteins. I g alpha and I g beta are part of the signaling complex in B cells and are phosphorylated by the Src-family kinase Lyn.

Describe how the following experimental manipulations were used to determine antibody structure. Reduction and alkylation of the antibody molecule

Reduction enabled breakage of disulfide bonds between the h and l chains and the pairs of h chains in the I g G molecule. Alkylation ensured that bonds between the separated chains would not reform.

Describe how phosphorylation of Src-family kinases can deliver both activating and inhibitory signals to Src kinases

Src-family kinases have two tyrosine sites on which they can be phosphorylated: an inhibitory and an activatory site.

Describe one way in which the structure of antibodies is superbly adapted to their function.

The antibody is a Y-shaped molecule with the antigen- binding regions located at the tips of the Y. At the junction is a flexible hinge region that allows the two tips to move with respect to one another and hence to bind to antigenic determinants arranged at varying distances from one another on a multivalent antigen.

Name one protein shown to be defective in many cases of X-linked agammaglobulinemia, and describe how a reduction in the activity of this protein could lead to immunodeficiency.

The signaling kinase Bruton's tyrosine kinase (Btk) is defective in 85% of cases of X-linked agammaglobulinemia.

Immunosuppressant drugs such as cyclosporin act via inhibition of the calcineurin phosphatase. If N FAT is ubiquitous, how do you think these drugs might act with so few side effects on other signaling processes within the body?

There are many possible answers to this question. For example, there may be multiple forms of NFAT, and so these drugs may just interact with particular forms of the enzyme.

True or false? Explain your answers. Interactions between receptors and ligands at the cell surface: can result in the creation of new covalent interactions within the cell.

True. Receptor-ligand binding can activate the receptor and result in phosphorylation and receptor cross-linking.