Slide 9.4

Internal Signaling Complexes and Their Role in Robustness

  • Internal signaling complexes increase robustness
  • Specific domains bind SH3 (SRC Homology 3) rich regions.

Adaptation in Signaling

  • Adaptation functions as a change detector allowing for desensitization.
  • This concept parallels other mechanisms influencing downstream effectors.

GPCRs (G-Protein Coupled Receptors)

  • Two primary pathways to turn GPCRs off:
    • Effect on G-protein:
    • G-proteins are active when bound to GTP and inactive when GDP is bound.
    • Key enzyme involved: RGS (Regulator of G-protein Signaling).
      • RGS acts as a GTPase-activating protein (GAP) specific to the alpha subunit of G-proteins.
      • Role of RGS:
      • Promotes hydrolysis of GTP to GDP in G-proteins, thereby turning off the signaling.
      • Specifically, RGS influences Gi type G-proteins.
    • Effect on downstream signaling:
    • Gi proteins inhibit adenylate cyclases, while Gs proteins activate them.

Influence of Cyclic AMP (cAMP) and CREB (cAMP Response Element Binding Protein)

  • Activation of CREB:
    • CREB binds to the promoter region of genes activated by cAMP (cAMP Response Element).
    • Functions alongside CBP (CREB Binding Protein) for transcription activation.

G-Proteins and Phospholipid Signaling

  • Gq protein activation:

    • Activated by GPCRs, leading to activation of phospholipase C (PLC).
    • Phospholipase C beta (PLC-β) acts in olfactory neurons.
    • Role of PLC:
      • Produces inositol trisphosphate (IP3) and diacylglycerol (DAG) from phosphatidylinositol 4,5-bisphosphate (PIP2).

  • cGMP Signaling:

    • cGMP is generated from GTP by guanylate cyclase and is vital in the phototransduction pathway in the retina.
    • Decreased cGMP levels lead to hyperpolarization in photoreceptors.

Protein Kinase Activation Dynamics

  • cAMP-dependent Protein Kinase Activation:
    • Protein kinases can exist in different states: inactive (compact or open), calcium-calmodulin activated, or homogenous active states.
  • The regulatory segment of protein kinase can either be phosphorylated or unphosphorylated, affecting kinase activity.
  • The two inactive states (compact and open) differ by the availability of the regulatory segment.
  • Homogeneity vs. calcium presence:
    • E.g., calcium presence causes a structural change leading to activation.

Translocation and Signal Sequences

  • Discussion of start vs. stop sequences:
    • Internal signal sequences and stop transfer sequences influence the translocation dynamics of proteins across membranes.

Enzyme-Coupled Receptors Classification

  • Enzyme-coupled receptors are classified into six main classes:
    1. Receptor Tyrosine Kinases (RTK)
    • Comprise a kinase domain, dimerize, and trans-phosphorylate, activating signaling pathways.
    1. Tyrosine Kinase-Associated Receptors:
    • These send signals similarly to RTKs but are associated with tyrosine kinases instead of having them as part of the receptor.
    1. Receptor Guanylate Cyclases:
    • Involved in the production of cGMP.
    1. Receptor Serine/Threonine Kinases:
    • Example: Transforming Growth Factor Beta (TGF-β) receptor signaling that leads to phosphorylation and activation of transcriptional regulators.
    1. Histidine Kinase-Associated Receptors:
    • Typically found in bacteria and fungi; they utilize a two-component system for environmental sensing.
    • Histidine kinase receives signals and phosphorylates response regulators.

RAS Signaling Pathway

  • RAS GTPases:
    • Monomeric GTPases crucial for cell growth and division; frequently mutated in cancers (approx. 30% of tumors have hyperactive RAS).
    • Activation process:
    • Signal binds to RTK, triggering dimerization and autophosphorylation, thus activating RAS through guanine nucleotide exchange.
    • RAS activation leads into MAP kinase signaling cascades (MAP: Mitogen Activated Protein), resulting in cell division responses.

MAP Kinase Signaling Cascade

  • The cascade involves multiple levels of kinases:
    • MAPKKK (RAF) activates MAPKK (MEK), which in turn activates MAPK (ERK).
    • These kinases are serine/threonine kinases focusing on proliferation and survival signals in cells.

Cross-Talk Prevention and Activation Specificity

  • Scaffold proteins prevent unwanted signaling cross-talk between different pathways, allowing specific responses tailored to upstream signaling.

Phosphoinositide Metabolism

  • Phosphatidylinositol Kinase Pathways:
    • This includes lipid kinases phosphorylating phosphoinositides, influencing various intracellular processes including cell survival.
  • Phosphatidylinositol 3-kinase (PI3K) activation leads to key cellular survival pathways.

Summary of Signaling Complex Dynamics

  • Importance of integrative signaling:
    • Multiple proteins and complexes cooperate within pathways.
    • Kinases and phosphatases modulate activities of process-specific molecules like AKT in cell growth and survival contexts.