CHAPTER 2Overview of Cellular Physiology in Medical Physiology 53Ca2+-binding protein involved in contraction of skeletal muscle
(Chapter 5). Calmodulin contains 148 amino acid residues
(Figure 2–22) and has four Ca2+-binding domains. It is unique
in that amino acid residue 115 is trimethylated, and it is exten-
sively conserved, being found in plants as well as animals. When
calmodulin binds Ca2+, it is capable of activating five different
calmodulin-dependent kinases (CaMKs; Table 2–4), among
other proteins. One of the kinases is myosin light-chain kinase,
which phosphorylates myosin. This brings about contraction in
smooth muscle. CaMKI and CaMKII are concerned with syn-
aptic function, and CaMKIII is concerned with protein synthe-
sis. Another calmodulin-activated protein is calcineurin, a
phosphatase that inactivates Ca2+ channels by dephosphorylat-
ing them. It also plays a prominent role in activating T cells and
is inhibited by some immunosuppressants.
MECHANISMS OF DIVERSITY
OF Ca2+ ACTIONS
It may seem difficult to understand how intracellular Ca2+ can
have so many varied effects as a second messenger. Part of the
explanation is that Ca2+ may have different effects at low and
at high concentrations. The ion may be at high concentration
at the site of its release from an organelle or a channel (Ca2+
sparks) and at a subsequent lower concentration after it dif-
fuses throughout the cell. Some of the changes it produces can
outlast the rise in intracellular Ca2+ concentration because of
the way it binds to some of the Ca2+-binding proteins. In ad-
dition, once released, intracellular Ca2+ concentrations fre-
quently oscillate at regular intervals, and there is evidence that
the frequency and, to a lesser extent, the amplitude of those os-
cillations codes information for effector mechanisms. Finally,
increases in intracellular Ca2+ concentration can spread from
cell to cell in waves, producing coordinated events such as the
rhythmic beating of cilia in airway epithelial cells.G PROTEINS
A common way to translate a signal to a biologic effect inside
cells is by way of nucleotide regulatory proteins that are acti-
vated after binding GTP (G proteins). When an activating sig-
nal reaches a G protein, the protein exchanges GDP for GTP.
The GTP–protein complex brings about the activating effect
of the G protein. The inherent GTPase activity of the protein
then converts GTP to GDP, restoring the G protein to an in-
active resting state. G proteins can be divided into two princi-
pal groups involved in cell signaling: small G proteins and
heterotrimeric G proteins. Other groups that have similar
regulation and are also important to cell physiology include
elongation factors, dynamin, and translocation GTPases.
There are six different families of small G proteins (or small
GTPases) that are all highly regulated. GTPase activating
proteins (GAPs) tend to inactivate small G proteins by
encouraging hydrolysis of GTP to GDP in the central binding
site. Guanine exchange factors (GEFs) tend to activate small
G proteins by encouraging exchange of GDP for GTP in the
active site. Some of the small G proteins contain lipid modifi-
cations that help to anchor them to membranes, while others
are free to diffuse throughout the cytosol. Small G proteins
are involved in many cellular functions. Members of the Rab
family regulate the rate of vesicle traffic between the endo-
plasmic reticulum, the Golgi apparatus, lysosomes, endo-
somes, and the cell membrane. Another family of small GTP-
binding proteins, the Rho/Rac family, mediates interactions
between the cytoskeleton and cell membrane; and a third
family, the Ras family, regulates growth by transmitting sig-
nals from the cell membrane to the nucleus.
Another family of G proteins, the larger heterotrimeric G
proteins, couple cell surface receptors to catalytic units that
catalyze the intracellular formation of second messengers or
couple the receptors directly to ion channels. Despite the
knowledge of the small G proteins described above, the heter-
omeric G proteins are frequently referred to in the shortened
“G protein” form because they were the first to be identified.
Heterotrimeric G proteins are made up of three subunits des-
ignated α, β, and γ (Figure 2–23). Both the α and the γ sub-
units have lipid modifications that anchor these proteins to
plasma membrane. The α subunit is bound to GDP. When a
ligand binds to a G protein-coupled receptor (GPCR), this
GDP is exchanged for GTP and the α subunit separates from
the combined β and γ subunits. The separated α subunit
brings about many biologic effects. The β and γ subunits are
tightly bound in the cell and together form a signaling mole-
cule that can also activate a variety of effectors. The intrinsicFIGURE 2–22 Structure of calmodulin from bovine brain.
Single-letter abbreviations are used for the amino acid residues. Note
the four calcium domains (purple residues) flanked on either side by
stretches of α helix. (Reproduced with permission from Cheung WY:
Calmodulin: An overview. Fed Proc 1982;41:2253.)
ADQLTEEQIA
KFE
AE
FLSFEKDGNGTT IT
K
EGTV
MSLGQNPET
IMDQLEA
NEVDA
DGNGTIDEPF
F
LTMMA
RK
M
K
D
TDSE
E
E
I
REAFRVF
DKD
G
N
G
YISAAELR
H
MV
LNT
EG
K
L
TDEE
VD
E
M
I
R
NIAED
EGDG
V
N
Y
EE
MQVF
M
TAKRLCOOH10Ca
2030NH Ac140Ca130120100
110
(Me) 3
N9060 8040Ca7050Ca