Chapter 2:  Cells and within cells

Cells

Translation

Receptor Mechanisms

    Neurons

   Proteins, Protein Structure and Protein....

    Glia

   Epigenetics

   Protein folding in the cell

Within cells

   Transcription

   Post-translational modifications of proteins

   Noncoding RNAs

   Protein degradation in the cell - Autophagy

   Membranes and Membrane Proteins

   miRNAs and the brain

   Protein secretion / Secretory pathway

   The Exctracellular Matrix

First the principles.......

The principle of receptor signaling through G-proteins is that the ligand induces a change in the sturcture of the receptor such that the receptor-ligand complex can activate the G-protein. Intracellular loops of the receptors are responsible for the activation of the G-proteins. The G-proteins associated with receptor transduction are the so-called "trimeric" G-proteins because they are composed of three subunits, alpha, beta and gamma. The trimeric designation distinguishes the receptor-associated G-proteins from smaller intracellular "monomeric" G-proteins that are involved in vesicular traffic and other processes within the cell.

This exchange is the rate-limiting step which determines how fast the G-protein hydrolyzes GTP to GDP. The hydrolysis of GTP to GDP returns the G-protein to the inactive state. Here the inherent GTPase activity of the alpha subunit hydrolyzes GTP to GDP. In the GDP-bound form the alpha subunit can now bind with the beta/gamma subunit and a trimeric inactive complex is formed.

As outlined above, the G-protein catalyzes the hydrolysis of GTP, with the rate limiting step in the GTPase cycle being the exchange of GDP for GTP. The ligand-receptor complex (indicated as [L-R] in the figure to right) facilitates this exchange process. In the active GTP-bound form the alpha subunit has two choices:

1.  it can hydrolyze GTP to GDP (and thus return to the inactive state), or

2. it can combine with an inactive effector protein (Ei) and activate it (Ea).

Likewise the free beta/gamma subunit can interact with an effector (E') to take it from the inactive to the active state (E'i to E'a). At rest, the bulk of the G-proteins are in the inactive GDP-bound trimeric form.

G-proteins take their name from the alpha subunit (i.e. Gs possesses the alpha(s) subunit, Gi the alpha(i) subunit, etc). There is a specificity in the substrate preference of alpha subunits. For example, alpha(s) stimulates the effector enzyme adenylyl cyclase: the "s" stands for "stimulation". Likewise, the alpha(i) inhibits adenylyl cyclase: the "i" stands for "inhibition". In general, alpha(q) stimulates the enzyme phospholipase C beta (the origins of the designation q is unclear; in earlier literature this subunit was designated "p", presumably the "p" for phospholipase C beta).

There are at least 18 different alpha subunits (listed in figure to the right). On the basis of amino acid sequence homology these can be classified into 4 families, the alpha(s),alpha(12), alpha(i) and alpha(q) family.

Alpha subunits can have more than one effector (list to left). For example, alpha(i), besides inhibiting adenylyl cyclase,  has been reported to interact with and stimulate K⁺ channels on the membrane (thus causing hyperpolarization and inhibition of the cell). This same alpha subunit has also been reported capable of inhibiting  voltage-operated Ca²⁺ channels on the membrane. Thus, this subunit has general inhibitory effect on the cell, whether it works through adenylyl cyclase, K⁺ channels, or Ca²⁺ channels. The closely related family member alpha(o) is also reported to act on K+ channels and probably Ca²⁺ channels as well. Some of the G-proteins are involved in sensory transduction (transduction of sensory information such as light, smell or taste into receptor cell potentials). For example, alpha(t), which is in the G(i) family, activates a phosphodiesterase (an enzyme for the breakdown of cyclic nucleotides) in the rod cells of the eye.

Alpha(olf) is found in the olfactory system where it participates in signal transduction of odorants and alpha(gust) is found in the gustatory system where it is involved in signal transduction of taste.

The toxins produced by the bacteria causing cholera and whooping cough (cholera toxin and pertussis toxin, respectively) have been found to work by acting on G-proteins. An analysis of their action on G-proteins has led  to a better understanding of how these bacteria can disrupt normal body function. The toxins have also become a valuable research tool in determining if hormones and neurotransmitters work through G-protein mechanisms.

Beta/gamma subunits have less target specificity   

There are at least 5 beta subunits and 12 gamma subunits. In general, beta/gamma dimers have much less target specicity than alpha subunits (i.e. they interact with a large spectrum of effectors). The nonspecific beta/gamma effects are perplexing since activation of all G proteins gives rise to free beta/gamma dimers. It is perplexing because it is unclear what  the point is of having specific alpha targets, if the beta/gammas are acitvating everything. One possible explanation is that beta/gamma in general must be present in far higher concentrations than alpha in order to exert their effects. Thus, beta/gamma may provide a general readout for the cell of the total incoming information transmitted by G proteins. Alternatively, beta/gamma dimers may be generated in signaling domains on the membrane which possess only certain effectors (and thus inappropriate activations are impossible).

Finally, there is one beta/gamma target which warrents special attention, namely beta-adrenergic receptor kinase (beta-ARK),  renamed G protein-coupled receptor kinase (GRK). This effector provides an internal negative feedback loop to the working of G protein receptors in general.