Chapter 6: Cell Communication

The Structure of Biological Membranes

• Cells communicate by cell signaling, which consists of four main processes:
  • (1) synthesis, release, and transport of signaling molecules
  • (2) reception of information by target cells
  • (3) signal transduction, the process by which a receptor converts an extracellular signal into an intracellular signal and relays the signal, leading to a cellular response
  • (4) response by the cell, for example, some metabolic process may be altered.

Sending Signals

• Most neurons (nerve cells) signal one another by releasing chemical compounds called neurotransmitters.
• Hormones are chemical messengers in plants and animals. \n In animals they are secreted by endocrine glands, glands that have no ducts.
  • Most hormones diffuse into capillaries and are transported by the blood to target cells.
• Local regulators diffuse through the interstitial fluid and act on nearby cells.
  • This process is called paracrine regulation.
  • Histamine, growth factors, prostaglandins (a type of local hormone), and nitric oxide (a gaseous signaling molecule that passes into target cells) are examples of local regulators.

Reception

• Each type of receptor has a specific shape, and only the signaling molecule that fits the specific receptor can affect the cell.
  • A cell can have many different types of receptors and can make different receptors at different stages in its life cycle or in response to different conditions.
  • Different types of cells can have different types of receptors.
• When a signaling molecule binds to an ion channel–linked receptor, the ion channel opens or, in some cases, closes.
• G protein–linked receptors are transmembrane proteins that extend into the cytosol or outside the cell.
  • These receptors couple specific signaling molecules to signal transduction pathways inside the cell.
  • The tail of the receptor that extends into the cytosol has a binding site for a specific G protein, a regulatory protein that binds to GTP.
• Enzyme-linked receptors are transmembrane proteins with a binding site for a signaling molecule outside the cell and a binding site for an enzyme inside the cell.
  • Many enzyme-linked receptors are tyrosine kinases in which the enzyme is part of the receptor.
• Intracellular receptors are located in the cytosol or in the nucleus.
  • Their ligands are small, hydrophobic molecules that diffuse across the plasma membrane.

Signal Transduction

• Ion channel–linked receptors convert chemical signals into electrical signals.
  • The gates of many ion channels remain closed until ligands bind to them.
• Many enzyme-linked receptors are tyrosine kinases, enzymes that phosphorylate proteins.
  • Tyrosine kinase receptors activate several different signal transduction pathways.
  • In a protein kinase cascade, each molecule in the signaling pathway is phosphorylated by the preceding protein kinase in the chain.
  • The last protein kinase in the cascade activates the target protein by phosphorylation.
  • The target protein alters some process in the cell.
• G protein–linked receptors activate G proteins.
  • A G protein consists of three subunits.
  • It is linked to a molecule of guanosine diphosphate (GDP), a molecule similar to ADP but containing the base guanine instead of adenine.
  • When a ligand binds with the receptor, the GDP is released and is replaced by guanosine triphosphate (GTP).
  • Then one subunit of the G protein separates from the other two subunits.
  • The activated G protein may initiate a signal transduction pathway by binding with a specific protein in the cell.
  • Some G proteins directly activate enzymes that catalyze changes in certain proteins, leading to changes in cell function.
• Intracellular receptors are located in the cytosol or nucleus.
  • These receptors are transcription factors that activate or repress the expression of specific genes.
• In many signaling systems, the signaling molecule serves as the first messenger.
• Information is relayed by the G protein to a second messenger, an intracellular signaling molecule.
• When certain G proteins undergo a conformational change, they bind with and activate adenylyl cyclase, an enzyme on the cytoplasmic side of the plasma membrane.
  • Adenylyl cyclase catalyzes the formation of cyclic AMP (cAMP) from ATP.
  • Cyclic AMP is a second messenger that activates certain protein kinase enzymes that phosphorylate certain proteins.
  • The phosphorylated protein triggers a chain of reactions that lead to some response in the cell.
• Certain G proteins activate the membrane-bound enzyme phospholipase C.
  • This enzyme splits a phospholipid, PIP 2, into two products, inositol trisphosphate (IP3) and diacylglycerol (DAG).
  • IP3 is a second messenger that can donate phosphate groups to proteins.
  • IP3 binds to calcium channels in the endoplasmic reticulum, which causes the channels to open and release calcium ions into the cytosol.
  • DAG is a second messenger that activates certain protein kinase enzymes.
  • These enzymes phosphorylate a variety of target proteins.
• Calcium ions can also act as second messengers.
  • They typically combine with the protein calmodulin, which then affects the activity of protein kinases and protein phosphatases.

Responses to Signals

• In response to signaling molecules, ion channels open or close, enzyme activity changes, leading to metabolic changes and other effects, and specific genes are activated or repressed.
  • These responses can affect cell shape, cell growth, cell division, cell differentiation, and metabolism.
• Signal amplification is the process of enhancing the cell’s response to a signal as the signal is relayed through a signal transduction pathway.
  • Before it becomes inactive, each enzyme can catalyze the production of numerous product molecules.
• Signal termination is the process of inactivating the receptor and each component of the signal transduction pathway once they have done their jobs.
  • Signal termination allows molecules in the system to respond to new signals.

Evolution of Cell Communication

• Molecules important in cell signaling first evolved in prokaryotes.
  • G proteins, protein kinases, and phosphatases have been highly conserved and are part of most signaling pathways.