Multiplication  Biophysical Substrate  Transitions and Metastable States

Second Messenger Cascades

Most second-messenger systems that have been explored to date are initiated by ligands binding to receptors, such as an odorant binding to a $\beta$-adrenergic receptor in a chemosensor or a photon ``binding" to and isomerizing a rhodopsin molecule in the photoreceptor [Lamb and Pugh Jr. (1992),]. Here, the rate of G-protein activation will be directly proportional to the amount of bound ligand.

By proposing that voltage-gated ion channels initiate second-messenger cascades as a form of self-regulation. we assume that the voltage, rather than a ligand concentration, sets the rate of G-protein activation. The second-messenger system can inherit its voltage-dependency from that of the ion channels in one of two ways: if the G-protein acts as a state sensor, the activation rate will be an S-shaped function of voltage; if instead it acts as a transition sensor, the rate will be a bell-shaped function of voltage. A state sensor corresponds to a single-step G-protein activation process in which the G-protein complex associates with only one of the two states of the channel. A transition sensor requires an additional, intermediate step, which could consist of either of the following:

1.

A short-lived, metastable state $\text{O}^\ast$ that lies between the stable open and closed states. If the G-protein binds only to the metastable state, then activation of the second-messenger cascade will be proportional to the ``concentration" of channels in that state, as given by the equation above.

2.

Since both the $\alpha$ subunit and $\beta\gamma$ complex of the G-protein can act as catalyzers for second-messenger, the unbinding of the $\beta\gamma$ complex could function as a transition sensor, as illustrated in fig.  13 ).

Figure 13: Hypothetical interaction of ion channel protein in the open state with the $\beta\gamma$ complex of a G-protein. 

 

 

 

 
Figure  13  is a purely hypothetical `Rube-Goldberg'-type mechanism that involves a second-messenger cascade initiated by a GTP-binding protein. Four homologous domains form the $\alpha$-subunit of an ion channel protein, at the center of which is the pore or P-region which allows selected species of ions to cross the membrane. The $\beta\gamma$complex of certain heterotrimeric GTP-binding protein binds to the calcium ion channel protein (see, for example, [De Waard et al. (1997)]). One direct, known effect of this binding is to shift the voltage-dependency of the calcium channel being in the open state.

By postulating that indirect effects of G-protein binding are also possible, we now show how the G-protein could conceivably act as both a state and a transition sensor. Figure  13  shows the $\beta\gamma$ complex associating with the S4 transmembrane segment of one of the ion channel domains. Only when the G-protein is bound to the ion channel protein can GTP bind to the $\alpha$ subunit of the G-protein, activating the catalytic properties of the $\alpha$ subunit. The activated $\alpha$ subunit acts as a state sensor, since it is produced at a rate proportional to the occupancy of the open state.

Under application of a transmembrane voltage, the S4 segment is visualized as moving down across the membrane, carrying with it the gating charge. This conformational change closes the pore. If this event also causes the G-protein's $\beta\gamma$ components to simultaneously dissociate from the channel protein, then the rate at which this occurs is proportional to the rate of ion channel state transitions. If the $\beta\gamma$ has catalytic properties when it is independent of the $\alpha$ subunit, then this component acts as a transition sensor.

 

3/7/1998