AP Biology – Ms. Whipple BCHS

 The yeast, Saccharomyces cerevisiae, has two mating types, a and   Cells of different mating types locate each other via secreted factors specific to each type 1 1. Exchange of mating factors 2. Mating 3. New a/  cell a a   a/ 

 A signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response  Signal transduction pathways convert signals on a cell’s surface into cellular responses  Pathway similarities in plants & bacteria suggest that ancestral signaling molecules evolved in prokaryotes and were modified later in eukaryotes

 The concentration of signaling molecules allows bacteria to sense local population density – Quorum Sensing  Quorum Sensing allows bacterial populations to coordinate their behaviors so that they can carry out activities that are only productive when performed by a given number of cells in synchrony.  One example of this is a Biofilm, an aggregation of bacterial cells that derive nutrition from the surface they are on.

Gap junctions between animal cells Plasmodesmata between plant cells (a) Cell junctions (b) Cell-cell recognition

 In many cases, animal cells communicate using local regulators, messenger molecules that travel only short distances  The ability of a cell to respond to a signal depends on whether or not it has a receptor specific to that signal. No Receptor = No Response

Local signaling Target cell Secreting cell Secretory vesicle Local regulator diffuses through extracellular fluid. (a) Paracrine signaling(b) Synaptic signaling Electrical signal along nerve cell triggers release of neurotransmitter. Neurotransmitter diffuses across synapse. Target cell is stimulated.

Endocrine cell Blood vessel Hormone travels in bloodstream. Target cell specifically binds hormone. (c) Endocrine (hormonal) signaling In long-distance signaling, plants and animals use chemicals called hormones

 Epinephrine stimulates the breakdown of Glycogen in liver and muscle cells.  This is useful during a “fight or flight” response because it gives immediate energy to muscles for fighting or fleeing.

1. Epinephrine does not interact directly with the enzyme responsible for glycogen breakdown; an intermediate step or series of steps must be occurring inside the cell. 2. The plasma membrane is somehow involved in transmitting the signal.

Plasma membrane EXTRACELLULAR FLUID CYTOPLASM Reception Receptor Signaling molecule 1

 A signaling molecule binds to a receptor protein, causing it to change shape.  The binding between a signal molecule (ligand) and receptor is highly specific  A shape change in a receptor is often the initial transduction of the signal  Most signal receptors are plasma membrane proteins

Plasma membrane EXTRACELLULAR FLUID CYTOPLASM ReceptionTransduction Receptor Signaling molecule Relay molecules in a signal transduction pathway 2 1

 Cascades of molecular interactions relay signals from receptors to target molecules in the cell.  The molecules that relay a signal from receptor to response are mostly proteins  Like falling dominoes, the receptor activates another protein, which activates another, and so on, until the protein producing the response is activated  At each step, the signal is transduced into a different form, usually a shape change in a protein

 Signal transduction usually involves multiple steps  Multistep pathways can amplify a signal: A few molecules can produce a large cellular response  Multistep pathways provide more opportunities for coordination and regulation of the cellular response

Plasma membrane EXTRACELLULAR FLUID CYTOPLASM ReceptionTransduction Response Receptor Signaling molecule Activation of cellular response Relay molecules in a signal transduction pathway 3 2 1

 Cell signaling leads to regulation of transcription or cytoplasmic activities  The cell’s response to an extracellular signal is sometimes called the “output response”  Ultimately, a signal transduction pathway leads to regulation of one or more cellular activities  The response may occur in the cytoplasm or in the nucleus  Many signaling pathways regulate the synthesis of enzymes or other proteins, usually by turning genes on or off in the nucleus  The final activated molecule in the signaling pathway may function as a transcription factor

Figure Growth factor Receptor Reception Transduction CYTOPLASM Response Inactive transcription factor Active transcription factor DNA NUCLEUS mRNA Gene Phosphorylation cascade P

 Other pathways regulate the activity of enzymes rather than their synthesis  Signaling pathways can also affect the overall behavior of a cell, for example, changes in cell shape

Wild type (with shmoos)  Fus3  formin Mating factor activates receptor. Mating factor G protein-coupled receptor Shmoo projection forming Formin G protein binds GTP and becomes activated P P P P Formin Fus3 GDP GTP Phosphory- lation cascade Microfilament Actin subunit Phosphorylation cascade activates Fus3, which moves to plasma membrane. Fus3 phos- phorylates formin, activating it. Formin initiates growth of microfilaments that form the shmoo projections. RESULTS CONCLUSION Figure 11.17

 A Receptor for that signaling factor!!

 A molecule that specifically binds to another molecule. For example, a signaling factor binding with a receptor causing a shape change and cellular response.

30% 1%  Cell Surface proteins make up 30% of all human proteins but only 1% have been examined by x-ray crystallography.

 G protein-coupled receptors (GPCRs) are the largest family of cell-surface receptors  A GPCR is a plasma membrane receptor that works with the help of a G protein  The G protein acts as an on/off switch: If GDP is bound to the G protein, the G protein is inactive

Figure 11.7b G protein-coupled receptor Plasma membrane G protein (inactive) CYTOPLASM Enzyme Activated receptor Signaling molecule Inactive enzyme Activated enzyme Cellular response GDP GTP GDP GTP P i GDP

 Receptor tyrosine kinases (RTKs) are membrane receptors that attach phosphates to tyrosines  A receptor tyrosine kinase can trigger multiple signal transduction pathways at once  Abnormal functioning of RTKs is associated with many types of cancers

Figure 11.7c Signaling molecule (ligand) Ligand-binding site  helix in the membrane Tyrosines CYTOPLASM Receptor tyrosine kinase proteins (inactive monomers) Signaling molecule Dimer Tyr P P P P P P P P P P P P Activated tyrosine kinase regions (unphosphorylated dimer) Fully activated receptor tyrosine kinase (phosphorylated dimer) Activated relay proteins Cellular response 1 Cellular response 2 Inactive relay proteins 6 ATP 6 ADP

 A ligand-gated ion channel receptor acts as a gate when the receptor changes shape  When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na + or Ca 2+, through a channel in the receptor

 Intracellular receptor proteins are found in the cytosol or nucleus of target cells  Small or hydrophobic chemical messengers can readily cross the membrane and activate receptors  Examples of hydrophobic messengers are the steroid and thyroid hormones of animals  An activated hormone-receptor complex can act as a transcription factor, turning on specific genes

Hormone (testosterone) Receptor protein Plasma membrane DNA NUCLEUS CYTOPLASM EXTRACELLULAR FLUID

Hormone (testosterone) Receptor protein Plasma membrane Hormone- receptor complex DNA NUCLEUS CYTOPLASM EXTRACELLULAR FLUID

Hormone (testosterone) Receptor protein Plasma membrane Hormone- receptor complex DNA NUCLEUS CYTOPLASM EXTRACELLULAR FLUID

Hormone (testosterone) Receptor protein Plasma membrane Hormone- receptor complex DNA mRNA NUCLEUS CYTOPLASM EXTRACELLULAR FLUID

Hormone (testosterone) Receptor protein Plasma membrane EXTRACELLULAR FLUID Hormone- receptor complex DNA mRNA NUCLEUS CYTOPLASM New protein

 Probably on the Plasma Membrane because hydrophilic (water soluble) molecules cannot easily get through the phospholipid bilayer.

 Possibility of greatly amplifying signal!!