Question

Manic depression may be caused by overactiviry of certain central nervous system cells, perhaps caused by abnormally high levels of hormones or neurotransmitters which stimulate phospholipidbased signal transduction (c.g., from phosphatidylinositol [PI]). Lithium has been used for many years to treat manic depression. In the presence of \(\mathrm{Li}^{*}\), the PI system is slowed despite continued stimulation and cells become less sensitive to these stimuli. Li \(^{+}\) may have two functions, inhibition of the phosphatase that dephosphorylates inositol trisphosphate and direct interference with the function of G proteins. Which of the following statements concerning G proteins is correct? A. G proteins bind the appropriate hormone at the cell surface. B. GTP is bound to G protein in the resting state. C. \(\alpha\) -Subunit may be cither stimulatory or inhibitory because it has two forms. D. Adenylate cyclase can be activated only if \(\alpha\) - and \(\beta\) -subunits of \(G\) protein are associated with each other. E. Hydrolysis of GTP is necessary for G protein subunits to separate.

Short answer

Answer: C. α-Subunit may be either stimulatory or inhibitory because it has two forms.

Step-by-step solution

Understand G Proteins

G proteins are protein molecules that transmit signals from various stimuli and receptors outside a cell to the cell's interior. They play a large role in signal transduction and are made up of three subunits: α, β, and γ.

Analyze Option A

A. G proteins bind the appropriate hormone at the cell surface. This statement is incorrect because G proteins do not directly bind hormones. Instead, hormones bind to specific receptors on the cell surface, which then activates the G protein.

Analyze Option B

B. GTP is bound to G protein in the resting state. This statement is incorrect. In the resting state, GDP (guanosine diphosphate) is bound to the α-subunit of the G protein. GTP (guanosine triphosphate) binds when the G protein is activated, replacing the GDP.

Analyze Option C

C. \(\alpha\)-Subunit may be either stimulatory or inhibitory because it has two forms. This statement is correct. There are indeed two forms of the α-subunit: Gαs (stimulatory) and Gαi (inhibitory). These subunits regulate the activity of different effector molecules, either activating or inhibiting them.

Analyze Option D

D. Adenylate cyclase can be activated only if α- and β-subunits of G protein are associated with each other. This statement is incorrect. In fact, adenylate cyclase is activated by the Gαs subunit alone when it is activated and dissociated from the β and γ subunits.

Analyze Option E

E. Hydrolysis of GTP is necessary for G protein subunits to separate. This statement is incorrect. Hydrolysis of GTP on the α-subunit is necessary for ending the signaling process and returning the G protein to its inactive state, rather than separating the subunits. Thus, the correct statement concerning G proteins is: C. \(\alpha\)-Subunit may be either stimulatory or inhibitory because it has two forms.