Chapter 11: Cell-Cell interactions (lect 10/17)

Types of Gap Junctions and Cell Interactions

  • Different types of gap junctions exist, significant for cell communication.

  • There are hundreds of models illustrating various connections between cells.

Plant Cell Walls

  • Plant cell walls prevent direct interactions between cells.

  • Two primary systems exist for cell communication within plants:

    • Symplast: Shared cytoplasm allowing for intracellular communication.

    • Apoplast: Extracellular space facilitating movement of substances.

Plasma Membrane and Intracellular Actions

  • The plasma membrane serves to separate plant tissues, influencing intracellular interactions.

  • Intracellular actions are triggered by outside activity and are notably regulated by guanine nucleotides, especially through binding to TTP.

Signal Transduction Pathways

  • Overview:

    • The signal binds to a receptor, initiating a cascade of molecular events.

    • G proteins, when inactive, are bound to GDP (guanosine diphosphate) and upon receptor binding, they exchange GDP for GTP (guanosine triphosphate).

  • G Protein Coupled Receptor (GPCR) Mechanism:

    • GTP-bound G protein dissociates into two functional parts, triggering the production of a second messenger.

    • The second messenger activates intracellular responses.

Enzyme Linked Receptors

  • Signal transduction can occur via enzyme-linked receptors, which are primarily transmembrane proteins that catalyze reactions inside cells.

  • Receptor Tyrosine Kinases (RTKs):

    • RTKs are a well-known group of enzyme-linked receptors.

    • Mechanism:

    • Signal binds to RTK monomers leading to dimerization (the process of two monomers forming a dimer).

    • Dimerization triggers autophosphorylation, activating the RTK by exchanging GDP for GTP.

    • Active RTK starts a phosphorylation cascade, where activated kinases phosphorylate multiple substrates, amplifying the signal.

Phosphorylation Cascades

  • Initiated by mitogens, specifically mitogen-activated protein (MAP) kinases.

  • Result in conversion of extracellular signals into amplified intracellular messages.

  • Phosphorylation cascades enable diversification of the original signal, effectively flicking the biological switch on or off (illustrated by the light switch metaphor).

Gene Activation and Expression Changes

  • All cells contain the same DNA; however, different cell types perform distinct functions.

  • DNA is tightly wound around proteins called histones.

    • When genes are needed, localized parts of the DNA unwind and can be read for transcription.

  • Changes in gene expression involve alterations in protein activity as well as the activation of specific genes in response to environmental signals (e.g., plant hormones in reaction to drought conditions).

Hormonal Responses in Plants

  • In response to water scarcity, hormones can bind to receptors in guard cells.

  • Guard cells manage gas exchange, allowing plants to conserve water.

  • The hormonal signal coordinates tissue responses, such as growth adjustments in reaction to stressors.

Signal Termination

  • Cells possess inherent systems to turn off signaling pathways.

  • These systems produce signals to signal down-regulation, helping to restore balance after a response.

Pathway Interactions and Cross-Talk

  • Different pathways can influence each other:

    • For example, one pathway can inhibit or stimulate another.

  • Such cross-talk indicates complex regulatory networks involved in cellular decision-making processes.

Biofilm Formation in Microbial Communities

  • Some bacteria assist in the formation of biofilms, helping to anchor microbial communities.

  • As population density increases, motility becomes limited, necessitating communal adhesion (e.g., dental plaque formation).

  • Organized microbial structures coordinate through gene protein coupled receptors, enabling cells to work in concert.