occur within less than 50 ms, that the activation of the G-protein and its interaction
with the effector protein occur within 500 ms, that the formation of second mes-
sengers such as cAMP can take up to 20 s, that the interaction of the GRK-
phosphorylated receptor withb-arrestin can take up to 50 s and that a conforma-
tional change in b-arrestin may take up to 5 min. These results indicate that
G-protein activation is the rate-limiting step in GPCR signalling. Interestingly,
whereas full agonists activate the G-protein within 50 ms, partial agonists take
up to 1 s and inverse agonists take about 1 s to reduce the intrinsic activity of
a receptor. Such studies have also shown that some Gi-like G-proteins can initiate
a signal without the need to dissociate whereas other G-proteins do appear to need
to dissociate first.17.4.4 Signal transduction through receptors with intrinsic protein
kinase activity
It has been appreciated for over 20 years that phosphorylation coupled with dephos-
phorylation represents an important mechanism for the regulation of protein activity.
A large number of intracellular kinases and phosphatases have been characterised
and their regulatory action linked to conformational changes induced in the target
protein as a result of the introduction or removal of a phosphate group. Control of
protein activity by the kinase/phosphatase principle is found in a broad range of
organisms, indicating its early evolution. It operates with the net consumption of ATP,
but with the considerable gain in sensitivity, amplification and flexibility that more
than compensate for the ATP consumed.
There are two main classes of receptor kinases based on the specificity of the
induced protein kinase, namely receptor tyrosine kinases and serine/threonine recep-
tor kinases.Receptor tyrosine kinases
Twenty subclasses of receptor tyrosine kinases (RTKs) are known, based on their extra-
and intracellular structures. The human genome contains approximately 90 RTK genes.
Three of the best characterised are the epidermal growth factor receptor (EGFR) (also
known as ErbB and HER), platelet derived growth factor (PDGF) receptor and
the insulin receptor (IR). The vast majority of RTKs, including EGFR and PDGF, are
single-chain, monomeric proteins in the absence of their agonist but they dimerise
on agonist binding. However, a few, including the insulin receptor and the closely
related insulin-like growth factor-1 (IGF-1) receptor, are permanently dimeric. Each
monomer of the insulin receptor consists of anaand abchain that are linked via a
disulphide bridge making the functional dimer ana 2 b 2 tetramer (Fig. 17.11). The
insulin receptor is involved in the regulation of lipid and protein metabolism in
addition to its role in the maintenance of glucose homeostasis. EGFR regulates aspects
of development including that of the nervous system, but mutant forms are linked
to cancer.698 Cell membrane receptors and cell signalling