trans

266 INTRACELLULAR SIGNALING
G11XG••G H125RDLKXXN T160 D185
R274
I II III IV V VIa VIb VII VIII IX X XI
4 26 44 59 74 99 122 139 158 176 210 255
K33 E51
D145FG A170PE
FIGURE 11.7 Structure of protein kinase catalytic domains. The 12 subdomains of the catalytic subunit are
shown, with residues important for catalytic activity indicated by arrows.
Structure of protein kinases
All protein kinases contain a specific domain
that catalyzes transfer of the -phosphate group
from ATP or GTP to the acceptor hydroxyl
group of serine, threonine or tyrosine residues.
The catalytic domain and its key catalytic
residues are well conserved amongst various
kinases. The catalytic domain folds into a twolobed
structure that consists of twelve subdomains
(Figure 11.7). The smaller amino terminal
lobe contains subdomains I–IV and the larger
carboxy terminal lobe contains subdomains
V–XI. ATP or GTP binds deep inside the cleft
between the two lobes, and is anchored by the
GXGXXGXV (glycine loop) consensus sequence
in subdomain I. The larger carboxy terminal
lobe is involved in peptide substrate recognition
and catalysis. Subdomain II contains an
invariant Lys that is involved in orientation of
the and phosphate groups of the purine
nucleotide. A salt bridge is formed between
this Lys and a conserved Glu in subdomain III.
Subdomain VIB contains the HRDLKXXN consensus
motif of the catalytic loop. The Asp
within this consensus motif coordinates phosphate
group transfer, while the Arg within this
motif stabilizes the loop. Subdomain VII also
folds into a strand-loop- strand structure
similar to subdomain VIB. The conserved DFG
motif is located in subdomain VII. The Asp
residue in the DFG motif helps to coordinate
with the Mg 2 or Mn 2 that connects with
- and -phosphates of the purine nucleotide.
The DFG motif is also at the beginning of the
activation domain for many kinases. The activation
segment usually ends with the APE
motif that faces the cleft region in subdomain
VIII. The invariant Arg in subdomain XI stabilizes
the large carboxy terminal lobe.
Mitogen-activated protein (MAP)
kinase-mediated signaling
MAP kinases (MAPKs) are involved in signal
cascades that typically activate gene transcription
and various other cellular events in
response to extracellular stimuli (Figure 11.8).
Metazoans and yeast contain a family of MAP
kinases for this purpose. External signals responsible
for MAPK activation associate with
catalytic receptor kinases, or with serpentine
receptors coupled to heterotrimeric G proteins.
Downstream of receptor activation, a variety
of proteins can be used to stimulate the MAP
kinase cascade, including Ras or Rac-like small
GTP-binding proteins, or STE20 kinases. The
catalytic and serpentine receptor pathways
converge by each activating Ras.
MAP kinases become activated when they
are phosphorylated on threonine and tyrosine
residues within the activation region of subdomain
VIII. These residues correspond to T183
and Y185 of the extracellular signal regulated
protein kinases 2 or ERK2, and p38. In the
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA