A heteromeric guanine nucleotide binding protein, also known as a heterotrimeric G protein, is a complex protein composed of three subunits, named α, β, and γ. This protein is involved in signal transduction pathways, primarily in response to extracellular stimuli such as hormones, neurotransmitters, and light.
The α subunit of the heteromeric G protein possesses intrinsic GTPase activity and can bind either guanosine diphosphate (GDP) or guanosine triphosphate (GTP). The β and γ subunits work together as a βγ complex and are often considered as a single functional unit.
When inactive, the heteromeric G protein is bound to GDP. Upon activation by a receptor, guanine nucleotide exchange occurs, leading to the exchange of GDP for GTP on the α subunit. This causes a conformational change in the G protein, leading to the dissociation of the α subunit from the βγ subunit.
Both the α-GTP subunit and the βγ subunit are then able to interact with various effector proteins to propagate the signal. The βγ subunit can directly modulate the activity of certain ion channels or enzymes, while the α-GTP subunit can interact with adenylyl cyclase or phospholipase C, triggering the production of second messengers like cyclic AMP or inositol trisphosphate.
The signal cascade initiated by heteromeric G proteins allows for the amplification and diversification of extracellular signals, enabling cells to respond to various stimuli and regulate a wide range of physiological processes.
