Structural elements and allosteric mechanisms governing regulation and catalysis of CSK-family kinases and their inhibition of Src-family kinases

Abstract

C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK) are endogenous inhibitors constraining the activity of the oncogenic Src-family kinases (SFKs) in cells. Both kinases suppress SFKs by selectively phosphorylating their consensus C-terminal regulatory tyrosine. In addition to phosphorylation, CHK can suppress SFKs by a unique non-catalytic inhibitory mechanism that involves tight binding of CHK to SFKs to form stable complexes. In this review, we discuss how allosteric regulators, phosphorylation, and inter-domain interactions interplay to govern the activity of CSK and CHK and their ability to inhibit SFKs. In particular, based upon the published results of structural and biochemical analysis of CSK and CHK, we attempt to chart the allosteric networks in CSK and CHK that govern their catalysis and ability to inhibit SFKs. We also discuss how the published three-dimensional structure of CSK complexed with an SFK member sheds light on the structural basis of substrate recognition by protein kinases.

c-Src and c-Yes are Two Unlikely Partners of Spermatogenesis and their Roles in Blood-Testis Barrier Dynamics

“…Tyr530 in c-Src and Tyr535 in c-Yes at the C-terminal region, however, are inhibitory in nature since their phosphorylation lead to a blockade of the corresponding SFK activity. Endogenous negative regulators of SFKs, namely C-terminal Src kinase (Csk) and Csk-homologous kinase (Chk), keep SFKs in an “off ” state in the absence of stimulation 48 and a conformational change as shown in Figure 2B would allow phosphorylation of the stimulatory tyrosine to activate SFKs. 33 It has been reported that the association of c-Src with the plasma membrane is relatively dynamic and is regulated by its intrinsic kinase activity and its SH2 domain 49 These observations thus indicate that SFKs are capable of interacting with a large number of partner proteins and substrates via their inherent SH domains, which pave the way for their ability to integrate and transmit diverse cellular signals.…”

Distal Loop Flexibility of a Regulatory Domain Modulates Dynamics and Activity of C-Terminal Src Kinase (Csk)

“…Further exploration of proximal regions via site-directed mutagenesis and alanine scanning studies has identified contacts that are deemed essential for kinase activation via the SH2 domain [21], [23]. As a modular enzyme, Csk's domain linkers also play key roles in domain packing and arrangement of the active kinase conformation [15]. A specific site in the SH2-kinase linker (residue 183), which packs against the small lobe of the kinase domain, is very sensitive to the size of its side chain (Phe in wild type Csk) and is essential for functional flexibility and Csk activation via the SH2 domain [24].…”

“…Stabilizing effects are seen in other regions that correspond to distal motifs and are part of the catalytic machinery. Such long-range coupled effects underlie the intrinsic cross-domain communications that are integral to Csk's function as a multidomain enzyme [15]. Notably, the conserved Arg313 of the catalytic loop ( i −1 to the catalytic base) is overall more stable in the variant trajectories than in the wild type.…”

“…A notable difference between SFKs and Csk is the tertiary arrangement of the SH domains [15] and the absence of activation loop phosphorylation in Csk. The kinase domain of Csk requires activation by the peripheral motifs and SH2 and SH3 domains, whereas these domains are important for localization and activity repression in SFKs via binding the phosphorylated tail [15].…”

“…A notable difference between SFKs and Csk is the tertiary arrangement of the SH domains [15] and the absence of activation loop phosphorylation in Csk. The kinase domain of Csk requires activation by the peripheral motifs and SH2 and SH3 domains, whereas these domains are important for localization and activity repression in SFKs via binding the phosphorylated tail [15]. Biochemical and structural characterization of SH2 and SH3 modules of many proteins is abundant in the literature as these domains have been utilized extensively to investigate protein folding and stability [16]–[17], thus providing a rich database that enables further nuanced exploration.…”