Most importantly, they all rely on specific knowledge of noncatalytic kinase function built over years of fundamental research


Most importantly, they all rely on specific knowledge of noncatalytic kinase function built over years of fundamental research. consider the contemporary landscape of small molecules to modulate noncatalytic functions of protein kinases, which, although challenging, has significant potential given the scope of noncatalytic protein kinase function in health IL-20R2 and disease. and within receptor-scaffolded dimers (29, 30, 31). While the mechanism is still debated (31), this function was clearly revealed by the discovery of activating pseudokinase domain name mutations (32), which promote JAK2 signaling and induce hematopoietic malignancies. Accordingly, from duplications of their kinase ancestors, pseudokinases can evolve pseudoactive sites that do not bind nucleotide, diminish their activation loops, and adopt conformations discordant with catalytic activity. Any of these modifications enable function as protein conversation domains that regulate activities of their cognate kinase partners allosterically. intermolecular interactions, kinases and pseudokinases are able to modulate the position of the key regulatory element, the C helix within the N-lobe of the kinase fold, to promote active or inactive conformations of the catalytically active partner kinase. Several distinct modes of dimerization have been reported to influence the position of C helix, which have been illuminated by detailed structural studies, and highlight the versatility of the kinase fold as a protein interaction domain (Fig.?3; (33, 34, 35)). Many of the different regulatory binding modes are illustrated by pseudokinase domain binding to a cognate kinase or pseudokinase domain, including: back-to-back (as observed for Ire1 and RNase L homodimers (36, 37), head-to-tail FX1 (as observed for EGFR family proteins, such FX1 as HER3 pseudokinase:EGFR kinase (38)), head-to-head (as found for IRAK3 homodimers and proposed for IRAK3 pseudokinase:IRAK4 kinase pairs (39)), and antiparallel side-to-side (exemplified for RAF:RAF kinase dimers and KSR pseudokinase:RAF kinase heterodimers (40, 41, 42)) modes. These studies raise the possibility that protein kinases may exert noncatalytic regulatory roles on other kinases, similar to those exerted by pseudokinases, as recently proposed for the parallel side-to-side mode of homodimerization reported for the granuloviral PK-1 kinase (34). While not yet observed among pseudokinase:kinase pairs, this binding mode couples dimerization with the C helix occupying a position synonymous with catalytic activity. Open in a separate window Figure?3 Modes of kinase dimerization. Examples of the five different modes of kinase dimerization described in the text. Structures displayed are EGFR:HER3 (PDB 4riw; (38)), CRAF (PDB 3omv; (41)), IRE1 (PDB 2rio; (36)), PK-1 (PDB 6vvg; (34)), and IRAK3 (PDB 6ruu; (39)), with the C-helix and activation loop depicted as in each. Furthermore, while currently poorly understood, some pseudokinases have been reported to allosterically regulate the activities of nonkinase enzymes, as proposed for VRK3 pseudokinase binding to, and activation of, the VHR phosphatase (43, 44). Overall, these findings illustrate the breadth of noncatalytic allosteric functions that can be mediated by pseudokinase domains and suggest these may be underappreciated functions of protein kinases more generally. Deducing the precise nature of these noncatalytic allosteric functions of conventional protein kinases remains a major challenge. Such studies FX1 will rely on elegant chemical biology and catalytically dead knockin approaches, rather than gene deletion or knockdown, to reveal functions beyond phosphoryl transfer. Kinases and FX1 pseudokinases as molecular switches Over the past 30?years, crystal structures of kinase and pseudokinase domains have captured the N- and C-lobes and FX1 the regulatory elements, the C helix and activation loop, and structural pillars of hydrophobic networks (termed spines) in a continuum of conformations, illustrating their intrinsic dynamicity (45, 46, 47). In the case of conventional, active kinases, this flexibility has been associated with regulation of catalytic activity. Basally, the apoenzyme is proposed to exist in a catalytically uncommitted state until ATP binding, which galvanizes the proteins internal hydrophobic networks and poises the kinase for catalysis. Allosteric effectors and oligomerization are known to modulate adoption of a catalytically active conformation signified by an intact regulatory (R)-spine and C helix Glu engaged in a salt bridge.