Which of the following statements is not true about G proteins?

(a) G proteins are involved in signal cascades

(b) G proteins become activated when bound to GDP

(c) Guanine nucleotides regulate G proteins

(d) None of the above

The statement that is not true is (b) G proteins become activated when bound to GDP.

G proteins are "off" or inactive when they are bound to Guanosine Diphosphate (GDP). The core of their function is a regulated cycle where they switch between an inactive GDP-bound state and an active GTP-bound state.

The G Protein Activation Cycle

1. Inactive State: In its resting state, the G protein's alpha subunit is bound to a GDP molecule.
2. Activation: When a signal molecule activates a G-protein-coupled receptor (GPCR), the receptor triggers the G protein to release its GDP. A molecule of Guanosine Triphosphate (GTP), which is abundant in the cell, then binds in its place.
3. Active State: The binding of GTP causes a conformational change in the G protein, switching it to its "on" or active state. This active protein then goes on to regulate downstream target proteins, continuing the signal cascade.

Therefore, G proteins are activated by binding to GTP, not GDP.