Principles of cell signaling - UT Southwestern

39057_ch14_cellbio.qxd 8/28/06 5:11 PM Page 622
SUBSTRATE 1
(Protein SER)
H
OH
PRODUCT 1
(Phosphorylated protein)
H
N
C
C
N
C
N
C
C
N
C
H
O
CH 2
H
O
H
O
CH 2
H
O
Protein kinases are two substrate enzymes
O - P O -
O
+
O - P O
O
SUBSTRATE
(Mg 2+. 2
ATP)
Mg 2+
O - P O
O
O - O -
O - P O
O
O - P O
O
CH 2
(Mg 2+. Triphosphate)
Ribose
PROTEIN KINASE
PRODUCT
(Mg 2+. 2
ADP)
Mg 2+
O - P O
O
CH 2
Adenine
Adenine
O
(Mg 2+. Diphosphate)
Rib
FIGURE 14.29 Protein kinases transfer the -phosphoryl group from ATP to
serine, threonine, or tyrosine residues in protein substrates.
Protein phosphorylation is the most common
form of regulatory posttranslational modification.
It occurs in all organisms, and it is estimated
that about one-third of proteins in animals are
at some time phosphorylated. Phosphorylation
can stimulate or inhibit the catalytic activity of
an enzyme, the affinity with which a protein
binds other molecules, its subcellular localization,
its ability to be further covalently modified,
or its stability. Single phosphorylations may cause
500-fold or greater changes in activity, and proteins
are often phosphorylated on multiple
residues in complex and interacting patterns.
Most protein phosphorylation in eukaryotes,
and essentially all in animals, is catalyzed
by protein kinases; dephosphorylation is catalyzed
by phosphoprotein phosphatases. Both
classes of enzymes are controlled by diverse
mechanisms. In addition, proteins are often
phosphorylated by multiple protein kinases, resulting
in the generation of a range of activity
states. This complexity allows inputs from different
signaling pathways to be integrated into
the resulting activity of the target.
In bacteria, plants, and fungi, an additional
protein phosphorylating system known as twocomponent
signaling is vital. The protein kinases
involved in two-component signaling are
unrelated to the eukaryotic protein kinase superfamily
and phosphorylate aspartate residues
rather than serine, threonine, or tyrosine.
Protein kinases transfer a phosphoryl group
from ATP to Ser, Thr, and Tyr residues of protein
substrates to form chemically stable phosphate
esters, as shown in FIGURE 14.29. In
animals, the distribution of phosphate among
these three amino acid residues is uneven:
~90%-95% is on Ser, 5%-8% on Thr, and less
than 1% on Tyr residues. The human genome
contains approximately 500 genes that encode
protein kinases, and many protein kinase
mRNAs undergo alternative splicing. This makes
the protein kinase gene superfamily one of the
largest functional gene groups. The number and
diversity of these enzymes emphasize the great
and varied uses of protein kinases to regulate cellular
functions. Although some protein kinases
have a limited tissue and/or developmental distribution,
many are ubiquitously expressed.
Protein kinases are grouped according to
their residue specificity. Protein kinases that
phosphorylate Ser will usually also recognize
Thr, hence the name protein Ser/Thr kinase.
Multicellular organisms have protein Tyr kinases,
which only recognize Tyr. Dual specificity
protein kinases can phosphorylate Ser,
Thr, and Tyr in the appropriately restricted substrate
conformational context and are generally
the most selective of the protein kinases.
The analysis of the kinomes of several organisms
has led to a more elaborate grouping
derived from sequence relationships, shown in
FIGURE 14.30, that also reflects to some extent
on regulatory mechanisms and substrate specificity.
For example, the AGC group is named
for its founding members, cAMP-dependent
protein kinase (PKA), cyclic GMP-dependent
protein kinase (PKG), Ca2+, and phospholipiddependent
protein kinase (PKC). These protein
kinases are regulated by second messengers and
prefer substrates that contain basic residues near
the phosphorylation site.
In addition to substrate specificity for amino
acid residues, most protein kinases are also selective
for local sequence surrounding the substrate
site. Screening strategies have resulted in methods
to predict if proteins contain consensus substrate
sites for a wide variety of protein kinases.
Antibodies can be used to identify and roughly
quantitate protein phosphorylation at specific
sites in proteins. Beyond local recognition, protein
kinases may display marked substrate selectivity
among similar proteins based on overall
622 CHAPTER 14 Principles of cell signaling