Antigen receptors & signal transduction Flashcards
What is a cellular signal?
An event that triggers a cell to change its proliferative or activation state.
The receptors need to convert extracellular signals into intracellular biochemical events that result in a cellular response.
The initiation of signaling from a cell surface receptor may require ligand-induced clustering of receptors, called ______-_______, or may involve ________ _____ of the receptor induced by its association with ligand.
cross-linking; conformation change
1) What is a common early event in signal transduction?
2) What are the two types of tyrosine kinases?
1) A common early event in signal transduction is the PHOSPHORYLATION of a phosphate residue on a TYROSINE (most common in immune cells), serine, or threonine side chain in the cytosolic portion of a receptor or adaptor protein.
2) Non-receptor tyrosine kinases and receptor tyrosine kinases (RTKs)
1) What is the difference between non-receptor tyrosine kinases and receptor tyrosine kinases (RTKs)?
2) Which one is seen on antigen receptors on B and T lymphocytes?
1) Non-receptor tyrosine kinases: receptor’s cytosolic tail has no intrinsic catalytic activity so a SEPARATE intracellular tyrosine kinase phosphorylates the receptor or associated protiens.
RTKs: an intrinsic tyrosine kinase domain(s) located in the cytoplasmic tails of the receptors when they are cross-linked by multivalent extracellular ligands.
2) Non-receptor tyrosine kinases
Intracellular signals are often transmitted via multi-protein complexes. Signalling molecules are often composed of distinct domains.
What are the three main signalling protein domains?
1) SH2 domains
2) SH3 domains
3) Pleckstrin Homology (PH) domain
Match the signalling protein domains to their definitions:
1) SH2 domains
2) SH3 domains
3) Pleckstrin Homology (PK) domain
A) recognize specific PHOSPHOLIPIDS such as PIP3 found on the inner plasma membrane.
B) composed of 100 amino acids folded in a particular conformation, and they bind to phosphotyrosine containing peptides in certain proteins.
C) composed of 100 amino acids, and they mediate PROTEIN-PROTEIN interactions by binding to PROLINE-rich stretches in certain proteins.
SH2 domains: composed of 100 amino acids folded in a particular conformation, and they bind to phosphotyrosine containing peptides in certain proteins.
SH3 domains: composed of 100 amino acids, and they mediate PROTEIN-PROTEIN interactions by binding to PROLINE-rich stretches in certain proteins.
Pleckstrin Homology (PK) domain: recognize specific PHOSPHOLIPIDS such as PIP3 found on the inner plasma membrane.
What are adaptors and scaffolds?
Modular proteins used by signalling complexes in lymphocyte activation.
Adaptors/scaffolds function as MOLECULAR HUBS that physically link different enzymes/proteins into a signalling complex so those proteins can interact with each other.
Adaptor/scaffolds LACK ENZYMATIC ACTIVITY.
(T/F) Adaptor is usually bigger than a scaffold.
False. Adaptor is usually smaller than a scaffold.
During receptor activation, intracellular signalling proteins are recruited to the plasma membrane.
What are the three ways recruitment is achieved?
1) Binding to activated adaptors/scaffold proteins of the membrane
2) Small GTPases
3) Phosphorylation of membrane phospholipids which recruits PH domain containing proteins to the cell membrane
How is recruitment of intracellular signalling proteins to the plasma membrane achieved by binding to activated adaptors/scaffold proteins of the membrane?
A typical adaptor protein contains A FEW SPECIFIC DOMAINS that mediate protein-protein interactions. They contain PROLINE-rich stretches that can bind SH3 domains of other proteins and TYROSINE residues that can be phosphorylated for docking sites for other signaling molecules.
Scaffold proteins such as LAT (Linker for the Activation of T cells) are relatively large proteins that can become TYROSINE phosphorylated on multiple sites in order to recruit many different proteins.
Signal transduction can be visualized as a ________ ______ phenomenon.
Why?
Social Networking
An initial signal results in proteins being brought close to one another at designated hubs (adaptors), leading the activation of specific enzymes that eventually influence the activation of specific downstream transcription factors or induce other cellular events, such as actin polymerization.
How is recruitment of intracellular signalling proteins to the plasma membrane achieved by small GTPases?
Small GTPases exist in two states: GDP bound (inactive) or GTP bound (active). They’re often membrane associated.
GEFs (guanine exchange factors) convert GDP bound to GTP bound.
GEFs are signalling proteins that get recruited to signalling pathways by adaptors such as Grb2 during T cell activation.
SOS (a GEF) activates Ras (a GTPase).
(T/F) Many small GTPases are not membrane-associated.
False!
Many are membrane-associated!
How is recruitment of intracellular signalling proteins to the plasma membrane achieved by phosphorylation of membrane phospholipids?
The phospholipid, PIP2, is more than a structural phospholipid. It also ACTIVELY PARTICIPATES in CELL SIGNALLING.
PI3 Kinase, when activated during signalling, phosphorylates PIP2 to PIP3 which serves as a binding site for proteins bearing PH domains.
Amplification of intracellular signals can occur via many different mechanisms.
Briefly describe the two common mechanisms of signal amplification.
1) Kinase cascades: kinases successively phosphorylate and activate each other in a specific sequence.
2) Generation of small-molecule second messengers: activation of enzymes can produce second messengers which can diffuse throughout the cell and activate specific target proteins. Ca2+ is an example.
Immune receptors are made up of _______ membrane proteins of the ____________ superfamily that are involved in ligand recognition, that associate with other __________ signalling proteins that have unique _________-containing motifs in their cytoplasmic tails.
Integral; Immunoglobulin; Transmembrane; Tyrosine
*these tyrosine-containing motifs are the key motifs.
What is the difference between ITAMs and ITIMs?
ITAMs: Immunoreceptor Tyrosine-based Activating Motifs. These recruit the Syk/ZAP-70 family TYROSINE KINASE via SH2 domain and induce IMMUNE CELL ACTIVATION.
ITIMs: Immunoreceptor Tyrosine-based Inhibitory Motifs. These induce signalling of inhibiting cellular responses. Phosphorylated ITIMs recruit PHOSPHATASES and counteract ITAM activation.
Which one of the statements is false?
1) Members of the immune receptor family include antigen receptors on B cells and T cells, Fc receptors on myeloid cells and mast cells, and activating and inhibitory receptors on NK cells, T cells, and B cells.
2) B-cell/T cell receptors have very long cytoplasmic domains and thus do not need signalling receptors.
2!
B-cell/T cell receptors have very short cytoplasmic domains and thus need signalling receptors to transmit the signal.
There are two tyrosine residues on ITAMs that can be phosphorylated by ______ family kinases when immune receptors are activated.
Tyrosine phosphorylated ITAMs recruit a distinct kinase of the ________ family, which contains two tandem _____ domains that each bind to one of the two phosphorylated motifs on the ITAM.
Binding causes a ________ ____ that activates the kinase, leading to additional signalling events that drive immune cell ________.
Src
ZAP-70/Syk; SH2
Conformational change; activation
Briefly list the four features of T and B cell receptor signaling.
1) Signaling downstream of T and B cell antigen receptors is characterized by a SIMILAR SEQUENCE of events.
2) The strength of TCR and BCR signaling influence the responses of lymphocytes during their development and activation (weak interactions survive)
3) Antigen receptor signaling is fine-tuned and modulated
4) Antigen receptor signals may cooperate with signals from co-stimulatory receptors.
Signaling downstream of T and B cell antigen receptors is characterized by a SIMILAR SEQUENCE of events.
Describe this sequence of events.
1) ligand binding to receptors induce conformational change such as unfolding of the cytoplasmic tail of the receptor that may allow previously hidden tyrosine residues of ITAM motif to become available to be phosphorylated
2) Activated Src family kinase (Lck) phosphorylates tyrosines in the ITAMs of signalling proteins
3) Two phosphorylated tyrosines in a single ITAM are recognized by a Syk family tyrosine kinase (Syk or zap-70) that has tandem SH2 domains that each bind to the phosphotyrosine.
4) Binding of the Syk family kinase causes a conformational change that activates the kinase, leading to additional signalling events that drive immune cell activation.
