C2006/F2402 '07 -- Key to Recitation Problems #7

1.   A. cAMP & IP3. Ca⁺⁺ activates calmodulin, so activity of subunit A should depend on Ca⁺⁺ levels. Ca⁺⁺ is released from the ER by the action of IP3, so the level of Ca⁺⁺ (and level of activity of subunit A) should depend in turn on the level of IP3.
    Binding of cAMP activates Protein Kinase A. PKA phosphorylates subunit B of PKK, and phosphorylation usually alters the activity of target protein. So activity of PKK should depend on levels of cAMP as well as on levels of Ca⁺⁺.

Answers to B:     B-1. adenyl cyclase
                                B-2. Ca⁺⁺
                                B-3. subunit B
                                B-4. subunit A
Explanation:
For H1: H1 binds to a G-protein linked transmembrane receptor →  activation of a G protein → activation of adenylyl cyclase →  cAMP →  binding to PKA →  phosphorylation and activation of subunit B → activation of PKK.
    Phosphodiesterase destroys the cAMP, converting it to AMP and preventing activation of PKA. 
DAG and PLC are not involved here, so antagonists or inhibitors of those components have no effect.

For H2: H2 binds to a transmembrane receptor that activates the DAG/IP3 pathway, so the receptor must activate PLC.  Both a TK receptor or a G protein coupled receptor can trigger the IP3 pathway. (Only a GPCR can activate the cAMP pathway).

    A G protein coupled receptor can activate PKK through the DAG/IP3 pathway as follows: 
The receptor-H2 complex → activation of a G protein →  activation of phospholipase C →  generation of DAG and IP3. The DAG is not involved in signaling here, but the IP3 →  ER →  release of Ca⁺⁺ →  binding to subunit A (calmodulin) →  activation of PKK.
    PLC inhibitors block generation of IP3, thereby blocking generation of Ca⁺⁺ and activation of calmodulin. Antagonists of DAG have no effect as DAG (& PKC) are not involved here; phosphodiesterase has no effect as cAMP is not involved either.

    A TK receptor could work like this: H2 could bind to a TK receptor →  activation of the receptor → self phosphorylation of cytoplasmic domain →  activation of phospholipase C (by binding of PLC to the receptor or to adapter proteins that bind to the receptor). Once PLC is activated, the rest of the pathway is the same.

Answers to C:     C-1. Both
                            C-2. One or the other but not both.  
                            C-3. Hormone 2.

Explanation for C-1 & C-2. A G protein can activate either the cAMP pathway  &/or the DAG/IP3 pathway. So both hormone receptors could be coupled to G proteins.  However the two receptors must activate different G proteins, as the two hormones activate different enzymes -- PLC vs adenylyl cyclase.  The same G-protein can activate both the IP3 & cAMP pathways (if it binds to & activates both PLC and AC).  However in this case, one hormone produces only the IP3 response, and the other hormone produces only the cAMP response. So it must be two different G proteins -- one that activates PLC and one that activates AC.

Explanation for C-3: Hormone 2 activates the IP3/DAG pathway. Either a G protein coupled receptor (which is a multipass protein) or a TK linked receptor (which is a single pass protein) can activate this pathway. Hormone 1 activates the cAMP pathway; only a GPCR (multipass) can activate this pathway.