Ion channels as targets for G-proteins

G-protein-coupled receptors can control ion channel function directly by mechanisms that do not involve second messengers such as cAMP or inositol phosphates. This was first shown for cardiac muscle, but it now appears that direct G-protein-channel interaction may be quite general. Early examples came from studies on potassium channels. In cardiac muscle, for example, mAChRs are known to enhance K+ permeability (thus hyperpolarising the cells and inhibiting electrical activity). Similar mechanisms operate in neurons, where many inhibitory drugs such as opiate analgesics reduce excitability by opening potassium channels. These actions are produced by direct interaction between the βγ subunit of G0 and the channel, without the involvement of second messengers.

The Rho/Rho kinase system

This recently discovered signal transduction pathway is activated by certain GPCRs (and also by non-GPCR mechanisms), which couple to G-proteins of the G12/13 type. The free G-protein α subunit interacts with a guanosine nucleotide exchange factor, which facilitates GDP-GTP exchange at another GTPase, Rho. Rho-GDP, the resting form, is inactive, but when GDP-GTP exchange occurs, Rho is activated, and in turn activates Rho kinase.

Rho kinase phosphorylates many substrate proteins and controls a wide variety of cellular functions, including smooth muscle contraction and proliferation, angiogenesis and synaptic remodelling. By enhancing hypoxia-induced pulmonary artery vasoconstriction, activation of Rho kinase is thought to be important in the pathogenesis of pulmonary hypertension.

(Inactive form)

Gq

Rho-GEF

Rho-GAP

Rho-GDPRho-GDI

Rho-GTP

MLC

MCLK

PLC

Hypercontraction

PKCIP3

G12/13

++

+

-

+

Actomyosin interaction

Ca2+

Ca2+

CPI-17 CPI-17

+

Agonists

ROCK Inhibitors

-ROCK

P

P

(Active form)

CaM

MBS M20 M20 CatCat

MBS M130M130

ADP ATP

P

MLC

MLCPhMLCPh

P

(Ang II, 5-HT, Thrombin, PDGF, ET-1, NE, Ach, Uro II, Histamine, Thromboxane A2, etc.)

The Rho-ROCK Signaling Pathway in Vascular Smooth Muscle Cell Contraction

Shimokawa and Rashid, TiPS 2007

Kinase-linked and related receptors.

These membrane receptors are quite different in structure and function from either the ligand-gated channels or the GPCRs. They mediate the actions of a wide variety of protein mediators, including growth factors and cytokines, and hormones such as insulin and leptin, whose effects are exerted mainly at the level of gene transcription.

Most of these receptors are large proteins consisting of a single chain of up to 1000 residues, with a single membrane-spanning helical region, associated with a large extracellular ligand-binding domain, and an intracellular domain of variable size and function. In many cases, the intracellular domain is enzymic in nature (with protein kinase or guanylyl cyclase activity).

The main types are as follow.

1. Receptor tyrosine kinases (RTKs). These receptors have the basic structure incorporating a tyrosine kinase moiety in the intracellular region. They include receptors for many growth factors, such as epidermal growth factor and nerve growth factor, and also the group of Toll-like receptors that recognise bacterial lipopolysaccarides and play an important role in the body's reaction to infection. The insulin receptor also belongs to the RTK class, although it has a more complex dimeric structure.

2. Serine/threonine kinases. This smaller class is similar in structure to RTKs but phosphorylate serine and/or threonine residues rather than tyrosine. The main example is the receptor for transforming growth factor (TGF).

3. Cytokine receptors. These receptors lack intrinsic enzyme activity. When occupied, they associate with, and activate, a cytosolic tyrosine kinase, such as Jak (the Janus kinase) or other kinases. Ligands for these receptors include cytokines such as interferons and colony-stimulating factors involved in immunological responses.

4. Guanylyl cyclase-linked receptors. These are similar in structure to RTKs, but the enzymic moiety is guanylyl cyclase and they exert their effects by stimulating cGMP formation. The main example is the receptor for ANF.

Signal transduction

Signal transduction generally involves dimerisation of receptors, followed by autophosphorylation of tyrosine residues. The phosphotyrosine residues act as acceptors for the SH2 domains of a variety of intracellular proteins, thereby allowing control of many cell functions.

They are involved mainly in events controlling cell growth and differentiation, and act indirectly by regulating gene transcription.

Two important pathways are:

1. the Ras/Raf/mitogen-activated protein (MAP) kinase pathway, which is important in cell division, growth and differentiation.

2. the Jak/Stat pathway activated by many cytokines, which controls the synthesis and release of many inflammatory mediators

In many cases, ligand binding to the receptor leads to dimerisation. The association of the two intracellular kinase domains allows a mutual autophosphorylation of intracellular tyrosine residues to occur. The phosphorylated tyrosine residues then serve as high-affinity docking sites for other intracellular proteins that form the next stage in the signal transduction cascade. One important group of such 'adapter' proteins is known as the SH2 domain proteins (standing for Src homology, because it was first identified in the Src oncogene product). These possess a highly conserved sequence of about 100 amino acids , forming a recognition site for the phosphotyrosine residues of the receptor. Individual SH2 domain proteins, of which many are now known, bind selectively to particular receptors, so the pattern of events triggered by particular growth factors is highly specific.

Ras/Raf/mitogen-activated protein (MAP) kinase pathway

Jak/Stat pathway