More signal transduction Cell-cell contact via gap junctions or plasmodesmata; signal molecule passes between cells Ce

1. More signal transduction Cell-cell contact via gap junctions or plasmodesmata; signal molecule passes between cells Cell-cell recognition molecule binds to specific receptor; “receptor-ligand interaction”

3. These ligands are primary messengers What happens next?

4. When the ligand binds, other molecules are produced within the cell “second messengers” relay the signal from interior of cell May trigger cell activity and/or gene expression

5. Many different types of molecules can be ligands Hydrophilic molecules bind to membrane receptors. Receptor may be found on many different cells. Hydrophobic molecules bind to intracellular receptors steroids retinoids

6. Families of membrane receptors G-protein-linked receptors tyrosine kinase receptors ion-channels intracellular

7. G protein-linked receptors Have similar structures but different amino acid sequences Seven transmembrane -helices N-terminus on extracellular side, C-terminus on cytosolic side

10. Two major types of G proteins Large heterotrimeric G proteins mediate signal transductions Small monomeric G proteins help regulate the cytoskeleton

11.  subunit binds to GTP;  and  stay together

12. G is bound to GDP; complex is inactive When a ligand binds it (G) releases GDP and binds GTP G separates from other subunits. These can move freely along membrane and bind to enzymes or other proteins, activating them. Both the G and G can bind to and activate different cellular proteins G protein can hydrolyze the GTP and quickly revert to inactive state

13. Many different types of G proteins can interact with different cellular proteins Vision Smell Acetylcholine receptors: one “family” are ion channels (contraction), another are G protein receptors (relaxation) Developmental pathways What does G protein activation accomplish? Activation of second messenger system cAMP calcium Levels are elevated

14. Tyrosine kinase pathways (what is a kinase? Adds phosphate groups to a particular amino acid) Growth, proliferation, specialization “cytoplasmic tail” portion of receptor has several tyrosines. Proteins are found as separate peptides in inactive form? What activates it? You know by now!

15. Dimer forms upon aggregation. Tyrosines are phosphorylated via ATP hydrolysis “relay” proteins bind to tyrosines, become activated These can now move through the cell and activate other pathways

16. Several families of tyrosine kinases

18. Tyrosin kinases can activate many pathways simultaneously Think about it: growth, differentiation, etc. require many different activities G protein pathways are not as diffuse

19. Ion channels Ligand gated activated when a specific molecule binds example: acetylcholine receptor in postsynaptic cell Voltage gated stimulus is electrical, not chemical Both important in neervous system activity in

20. Na channel

22. Intracellular receptors In nucleus or cytosol Ligand is small and hydrophobic Promotes transcription or cell activation

24. Now that we have seen how the signal activates the cell, how is the signal actually transduced?