Polymodal Allosteric Regulation of Type 1 Serine/Threonine Kinase Receptors via a Conserved Electrostatic Lock

Type 1 Serine/Threonine Kinase Receptors (STKR1) transduce a wide spectrum of biological signals mediated by TGF-β superfamily members. The STKR1 activity is tightly controlled by their regulatory glycine-serine rich (GS) domain adjacent to the kinase domain. Despite decades of studies, it remains unknown how physiological or pathological GS domain modifications are coupled to STKR1 kinase activity. Here, by performing molecular dynamics simulations and free energy calculation of Activin-Like Kinase 2 (ALK2), we found that GS domain phosphorylation, FKBP12 dissociation, and disease mutations all destabilize a D354-R375 salt-bridge, which normally acts as an electrostatic lock to prevent coordination of adenosine triphosphate (ATP) to the catalytic site. We developed a WAFEX-guided principal analysis and unraveled how phosphorylation destabilizes this highly conserved salt-bridge in temporal and physical space. Using current-flow betweenness scores, we identified an allosteric network of residue-residue contacts between the GS domain and the catalytic site that controls the formation and disruption of this salt bridge. Importantly, our novel network analysis approach revealed how certain disease-causing mutations bypass FKBP12-mediated kinase inhibition to produce leaky signaling. We further provide experimental evidence that this salt-bridge lock exists in other STKR1s, and acts as a general safety mechanism in STKR1 to prevent pathological leaky signaling. In summary, our study provides a compelling and unifying allosteric activation mechanism in STKR1 kinases that reconciles a large number of experimental studies and sheds light on a novel therapeutic avenue to target disease-related STKR1 mutants.
Author summary Kinases play central role in essential physiological process and are attractive therapeutic drug targets. One of the important kinase families is Type 1 Serine/Threonine Kinase Receptors (STKR1), which control gene expression in response to extracellular growth factors. The activities of STKR1 are tightly controlled by their regulatory domain, which is distant from the kinase catalytic site. The underlying molecular mechanism is elucidated here. We identified that formation or disruption of a highly conserved charge-charge interaction located near the ATP binding site, mediates the physiological inhibition or activation of STKR1. We find that the stability of this charge-charge interaction is remotely controlled by interactions propagated from the distant regulatory domain. Several disease-causing mutations are located at the regulatory domain. We demonstrate how those mutations bypass these endogenous STKR1 inhibition mechanisms to produce pathological phenotypes. This study provides a general activation mechanism in STKR1 kinases, thus may benefit understanding the molecular mechanism of diseases and drug development.