to the active state [83]. The main role of the cyclin interaction is to push αC towards
the active site, while phosphorylation stabilizes an active conformation of the
activation segment by interaction with the HRD arginine residue as described
above (Fig. 4d).
In analogy to the role of the pseudokinase HER3 in EGFR activation, other
catalytically inactive (pseudo) kinases have been shown to activate catalytically
competent kinases in trans. Examples include the activation of JAK family
members by N-terminal pseudokinase domains [84] as well as the activation of
LKB1 (liver kinase B1) by the pseudokinase “STE20-related adaptor protein”
(STRADα or STRADβ) [85]. STRAD forms a heterotrimeric complex with
LKB1 and the scaffolding protein MO25 which dramatically enhances the activity
of LKB1, a kinase that does not require activation segment phosphorylation [86].
Despite the lack of catalytic activity, STRAD is still capable of binding
ATP with high affinity. The structure of the trimeric complex revealed that MO25
and ATP binding stabilizes an active-like state of STRAD in which MO25 stabilizes
active conformation of αC in a similar way as reported for CDK2. This interaction
was also observed in dimeric complexes of MO25 with active kinases [87].
The active-like conformation of STRAD is required for LKB1 activation which is
achieved by tight contacts of STRAD with the LKB1 substrate binding site [88, 89].
4 Canonical Type-I and Type-II Inhibitor Binding Mode
The plasticity of the kinase catalytic domain that is essential for kinase regulation
also offers an opportunity of targeting structurally diverse states. The most
explored design strategies are inhibitors targeting the active state (type-I inhibitors),
as well as inhibitors that target the so-called DFG-out conformation (type-II
inhibitors), an inactive conformation of the DFG motif that leads to an additional
large pocket. Since the active state is most stable, the largest fraction of known
structural models available in the protein data bank represent the type-I binding
mode. Type-I inhibitors are ATP mimetics, thus similar to the adenosine ring of
ATP, they form 1-3 hydrogen bonds with the main chain backbone of the kinase
hinge region. A large number of typically heterocyclic aromatic mono- to tricyclic
ring systems have been explored as ATP mimetic type-I scaffolds. As the active
state is highly conserved, selectivity of many type-I inhibitors is low. However,
shape complementarity and sequence variations can be used for the development
of inhibitors with high or restricted selectivity (Fig. 5a).
For instance, unique sequence features flanking the hinge, such as rare amino
acids in hydrophobic regions and variable structural elements, have been shown
to increase selectivity of type-I inhibitors. Noteworthy is the gatekeeper residue,
which controls the access to the hydrophobic back cavity. Small gatekeeper residues
such as threonine provide access to a larger back cavity and are present in only about
5% of all kinases. Type-I inhibitors targeting this hydrophobic site have therefore
favourable selectivity profiles by excluding ATP sites with bulkier gatekeeper
10
S. Röhm et al.
the active site, while phosphorylation stabilizes an active conformation of the
activation segment by interaction with the HRD arginine residue as described
above (Fig. 4d).
In analogy to the role of the pseudokinase HER3 in EGFR activation, other
catalytically inactive (pseudo) kinases have been shown to activate catalytically
competent kinases in trans. Examples include the activation of JAK family
members by N-terminal pseudokinase domains [84] as well as the activation of
LKB1 (liver kinase B1) by the pseudokinase “STE20-related adaptor protein”
(STRADα or STRADβ) [85]. STRAD forms a heterotrimeric complex with
LKB1 and the scaffolding protein MO25 which dramatically enhances the activity
of LKB1, a kinase that does not require activation segment phosphorylation [86].
Despite the lack of catalytic activity, STRAD is still capable of binding
ATP with high affinity. The structure of the trimeric complex revealed that MO25
and ATP binding stabilizes an active-like state of STRAD in which MO25 stabilizes
active conformation of αC in a similar way as reported for CDK2. This interaction
was also observed in dimeric complexes of MO25 with active kinases [87].
The active-like conformation of STRAD is required for LKB1 activation which is
achieved by tight contacts of STRAD with the LKB1 substrate binding site [88, 89].
4 Canonical Type-I and Type-II Inhibitor Binding Mode
The plasticity of the kinase catalytic domain that is essential for kinase regulation
also offers an opportunity of targeting structurally diverse states. The most
explored design strategies are inhibitors targeting the active state (type-I inhibitors),
as well as inhibitors that target the so-called DFG-out conformation (type-II
inhibitors), an inactive conformation of the DFG motif that leads to an additional
large pocket. Since the active state is most stable, the largest fraction of known
structural models available in the protein data bank represent the type-I binding
mode. Type-I inhibitors are ATP mimetics, thus similar to the adenosine ring of
ATP, they form 1-3 hydrogen bonds with the main chain backbone of the kinase
hinge region. A large number of typically heterocyclic aromatic mono- to tricyclic
ring systems have been explored as ATP mimetic type-I scaffolds. As the active
state is highly conserved, selectivity of many type-I inhibitors is low. However,
shape complementarity and sequence variations can be used for the development
of inhibitors with high or restricted selectivity (Fig. 5a).
For instance, unique sequence features flanking the hinge, such as rare amino
acids in hydrophobic regions and variable structural elements, have been shown
to increase selectivity of type-I inhibitors. Noteworthy is the gatekeeper residue,
which controls the access to the hydrophobic back cavity. Small gatekeeper residues
such as threonine provide access to a larger back cavity and are present in only about
5% of all kinases. Type-I inhibitors targeting this hydrophobic site have therefore
favourable selectivity profiles by excluding ATP sites with bulkier gatekeeper
10
S. Röhm et al.