A role for cAMP

Desensitization from persistent signal

Other second messengers

• Phospholipase C cleaves membrane lipid phosphatidylinositol 4,5 bisphospate into two messengers diacylgllycerol and inositol 1,4,5 trisphosphate (IP3)

• IP3 in turn activates release of calcium ions that act as a messenger and activate protein kinase C (numerous isozymes with tissue specific roles, for instance in cell division)

PLC mediated signal transduction

Regulation of cell cycle by protein kinases

Cyclin-dependent protein kinases control cell cycle

• By phosphorylating specific proteins at precise time intervals these kinases orchestrate the metabolic activities of the cell for cell division

• Heterodimers – one regulatory subunit (cyclin) and one catalytic subunit (cyclin-dependent protein kinase [CDK])_

Post-translational regulation through phosphorylation and proteolysis

Four mechanisms to control CDK activity

• Phosphorylation– Phosphorylate tyrosine prevents ATP binding– Removal of phosphate from tyrosine and

phosphorylation of threonine allows substrate binding

• Controlled degradation– Feedback loop involving DBRP

• Regulated synthesis of CDKs and cyclins– MAPK mediated activation of Jun and Fos

• Inhibition of CDK– Specific proteins such as p21 bind and inactivate CDK

Observe variations in the activities of specific CDKs during cell cycle

Whither MAPK?

Regulation of passage from G1 to S

Neuron function and signal transduction

• Voltage- and ligand-gated

• ion channels

Allosteric effectors of protein structure/function

Glutamate receptor

http://www.ibcp.fr/GGMM/Nimes/O11.html

Forming memories

• http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LTP.html

• Mini-review handout

• http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11807168&dopt=Abstract

Integrating circuits

• Circuits exhibit synergy within a cellular context• Bhalla and Iyengar modeling signal transduction

in the brain and long-term potentiation (LTP) (Fig 8.15)

• http://doqcs.ncbs.res.in/~bhalla/doqcs/template.php?x=home&y=index

• PKC activates MAPK, while MAPK helps activate PKC (Figure 8.16)

Why does it take 100 minutes of 5 nM EGF to reach LTP?

• 10 min at 5 nM or 100 min at 2 nM EGF is insufficient for LTP (Fig 8.18)

• Fig 8.19 result of determining concentration dependence of MAPK activation of PKC and the converse

• Three intersection points – MM 8.2 “Cobweb”– A indicates high activity for both enzymes– B indicates low activity for both– T is threshold stimulation, if EGF is sufficient to activate

either PKC or MAPK above T – both will reach A (T serves as a switch between A and B)

Turning off LTP

• Use a phosphatase to knock MAPK below threshold

• AA (arachidonic acid) generated by PLA2 persists, which makes it hard to turn off

• Takes awhile to de-phosphatase

Integrating more circuits

• Start with MAPK circuit

• Add calcium activation, etc.

• Result in Figure 8.23– PKC– MAPK– cAMP– Calcium

Discovery questions

• Chapter 8 31-35