IMMUNOKINASES - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 320

describe the principles underlying the activity of immuno-
kinases and recent work to develop an immunokinase
against Hodgkin lymphoma. The targeting moiety in this
case is the CD30 ligand (CD30L), which binds to the
internalizing receptor CD30 preferentially expressed on
Hodgkin lymphoma cells, and the kinase moiety is the
pro-apoptotic serine/threonine kinase DAPK2 [7].

21.2.2 Death-Associated Protein Kinases (DAPKs)

The eukaryotic protein kinases are divided into eight broad
classes based on catalytic domain structure, substrate spec-
ificity, and regulation mechanism [18,19] (Figure 21.1).
The calcium/calmodulin-dependent kinases (CAMKs)
include the death-associated protein kinases (DAPKs),
which are soluble, cytosolic kinases that play an important
role in the promotion of apoptosis in tumor cells [20-22].
DAPK1 was identified using a functional gene cloning
approach based on the transfection of mammalian cells
with an antisense cDNA library and the subsequent isolation
of antisense cDNA fragments that protected HeLa cells from
apoptosis induced by interferon- g [23]. This led to the
discovery of four additional members—DAPK2 (also
known as DRP-1), DAPK3 (also known as ZIP), DRAK-
1, and DRAK-2—all of which share significant sequence
and functional homology including the ability to induce
apoptosis when overexpressed [24]. Phylogenetic analysis
based on the alignment of catalytic domain sequences has
shown that DAPK1, DAPK2, and DAPK3 can be grouped
into a distinct clade with high bootstrap probabilities,
whereas DRAK1 and DRAK2 form another clade sharing
a putative common ancestor with other DAPK-related pro-
teins. Although the catalytic domains of the five DAPKs are

21.2 PROTEIN KINASES, APOPTOSIS, AND
CANCER

21.2.1 General Properties of Protein Kinases

Protein kinases are enzymes that add phosphate groups to
specific target proteins. Phosphate groups have a strong
negative charge, and their presence therefore has a profound
effect on the net charge of the modified protein and its
interactions with other molecules in the cell. Phosphorylation
by protein kinases is a useful mechanism to control protein
activity, and it is thought that more than 30% of all proteins
in mammalian cells are regulated in this manner [12].
Protein phosphorylation is usually reversible, with specific
protein phosphatases responsible for removing the phosphate
groups. The three basic components of the phosphorylation
system are therefore phosphoproteins whose properties are
altered by phosphorylation and dephosphorylation, protein
kinases that transfer phosphate groups from a donor substrate
such as ATP or GTP to target phosphoproteins, and protein
phosphatases that dephoshorylate phosphorylated proteins,
thereby restoring the protein phosphorylation system to its
nonphosphorylated state [13].
Because protein kinases are proteins that can themselves
be regulated by phosphorylation, kinase cascades are often
used to transduce signals within the cell, and protein phos-
phorylation is one of the most common mechanisms used to
convert extracellular signals into intracellular responses
[12]. Such signals are used to control every conceivable
cellular process, including cell growth, proliferation, migra-
tion, differentiation, and (most relevant in the context of this
chapter) apoptosis. Signaling must be tightly controlled to
avoid these fundamental processes going awry [14]. Both
mutations and epigenetic modifications can disrupt kinase-
mediated signaling by interfering with the expression and/or
activity of individual protein kinases, and this can have
profound effects on the core cellular processes listed above,
including the prevention of apoptosis in cells that would
normally be programmed to die. Such effects often contrib-
ute to the progression of cancer [8,15] and many kinases and
phosphatases have been identified as the products of onco-
genes and tumor suppressor genes [16]. The human genome
encodes 518 protein kinases, representing

FIGURE 21.1 Dendrogram of eukaryotic protein kinase fami-
lies based on sequence similarity within the catalytic domains.
Abbreviations: AGC, PKA, PKG and PKC kinases; CAMK,
calcium/calmodulin-regulated kinases; CK1, casein kinase 1;
CMGC, CDK, MAPK, GSK3, and CLK kinases; RGC, receptor
guanylate cyclases; STE, sterile phenotype (MAP kinase cascade
kinases, homologs of the yeast STE7, STE11, and STE20 kin-
ases); TK, tyrosine kinases; TKL, tyrosine kinase-like kinases.
Any protein kinases falling outside this classification are
described as atypical kinases. Source: From Reference 17.
Reprinted with permission from AAAS.

2% of all genes,
and many of these have been linked with cancer and other
diseases [17].