Bio 202 - Cell Communication

Cell Communication in Yeast

• yeast have two sexes, a and alpha haploid cell

• they release their own mating factors, amino acids which bind to receptors on the other sex

• the binding lets the cells detect each other, change shape, and fuse to create a diploid cell

Cell Communication in Bacteria

In a process called quorum sensing, they release a chemical sensor to monitor their total population density and then coordinate group behavior rather than acting like individual cells

Local Communication: direct contact

signals pass thru intercellular connection, or are transmitted thru cell-cell recognition when surface molecules bind

Local Communication: paracrine

signals released by a cell travel a short distance, influencing cells in the near vicinity

Long Distance Communication: hormonal/endocrinal

signals can move far throughout the body through the vascular system in plants and animals or through the air in some plants and insects

3 Stages of Cell Communication

1) Reception: a signal molecule called a ligand binds to a cell receptor
2) Transduction: a multi-step process called signal-transduction pathway occurs
3) Response: final response

3 stages using the example of Adrenaline

1) Reception: the ligand epinephrine binds to a cell
2) Transduction: activation and enzymatic activity occurs in the cell
3) Response: the enzyme glycogen phosphorylase is activated and assists in glycogen breakdown, creating energy

Ligand-gated ion channels

Ligand binds to a site on the receptor which causes conformational change. The protein opens its “gate” allowing specific ions through. The change ion concentration triggers a response.

examples: fast synaptic signaling

G protein-coupled receptor (GPCR)

A ligand binds to the GPCR, causing it to change shape. This activates a nearby G protein by allowing it to exchange its GDP for GTP. The activated G protein then moves along the membrane and binds to a target enzyme (like adenylyl cyclase), activating it and triggering secondary messengers which leads to a cellular response. The signal ends when the G protein hydrolyzes GTP back to GDP.

examples: Yeast Mating, Adrenaline

Tyrosine-Kinase Receptors

A ligand binds to two TKRs, causing them to dimerize (pair up). This activates their intracellular kinase domains (enzymatic ability), leading them to phosphorylate each other on specific tyrosine residues (autophosphorylation). The phosphorylated tyrosines serve as docking sites for signaling proteins, which bind and become activated, triggering a signaling cascade that produces a cellular response.

Internal Receptor

Some cells can allow signals to pass through the membrane. Their receptor is located inside the cell, and can carry out transduction within themselves without extra pathways

List the advantages of a multistep pathway in the transduction stage of cell signaling.

• Signal amplification: Each activated receptor can activate multiple intracellular signaling proteins, and each of those can activate many downstream molecules (e.g., second messengers). This allows a small amount of ligand to generate a large cellular response.

• Greater regulation and control: Multiple steps provide several points where the pathway can be enhanced or reduced

• Signal integration and coordination: Different signaling pathways can interact, allowing the cell to combine multiple signals and produce a coordinated, context-dependent response.

Phosphorylization Cascade

a sequence of signaling pathway events where one enzyme phosphorylates another using ATP, causing a chain reaction leading to the phosphorylation of thousands of proteins, speeding up and amplifying the original signal

- this process can be inhibited by enzymes called phosphatases (opposite: they remove phosphate groups)
- phosphatases turn off the cascade when there is no signal

Secondary Messengers

non-protein molecules that compose parts of the transduction step

cyclic AMP

Used in G-protein-linked-receptors — G-protein activates the enzyme adenylyl cyclase which causes a change of ATP into cAMP (second messenger). The formed cAMP then activates “kinase A” to continue the cascade to result in a cellular response.

Ca 2+ pathway

Used in both G protein and RTK’s — After receiving a first signal molecule, G protein activates phospholipase C which catalyzes the creation of the second messengers IP3 and DAG. IP3 specifically binds to a gated calcium channel on the endoplasmic reticulum which lets Ca 2+ ions diffuse into the cell to create a response

Outcomes of Cell Signaling

Cell A+D: When different cells respond differently to the same signal
Cell B: When a single receptor activates a pathway that splits into multiple downstream targets inside the same cell, leading to diff responses.
Cell C: When different signaling pathways converge on the same protein or transcription factor and activate the same response.

Cytosolic Cellular Response

A response that takes place in the cytoplasm

✓ No effect of gene expression

✓ Many steps (cascade = signal amplification)

Nuclear Cellular Response

Information is to the nucleus

✓ causes a change in gene expression

• typical of hormones or growth factors