SIRT2 deacetylase regulates the activity of GSK3 isoforms independent of inhibitory phosphorylation

Glycogen synthase kinase 3 (GSK3) is a critical regulator of diverse cellular functions involved in the maintenance of structure and function. Enzymatic activity of GSK3 is inhibited by N-terminal serine phosphorylation. However, alternate post-translational mechanism(s) responsible for GSK3 inactivation are not characterized. Here, we report that GSK3a and GSK3β are acetylated at Lys246 and Lys183, respectively. Molecular modeling and/or molecular dynamics simulations indicate that acetylation of GSK3 isoforms would hinder both the adenosine binding and prevent stable interactions of the negatively charged phosphates. We found that SIRT2 deacetylates GSK3β, and thus enhances its binding to ATP. Interestingly, the reduced activity of GSK3β is associated with lysine acetylation, but not with phosphorylation at Ser9 in hearts of SIRT2-deficient mice. Moreover, GSK3 is required for the anti-hypertrophic function of SIRT2 in cardiomyocytes. Overall, our study identified lysine acetylation as a novel post-translational modification regulating GSK3 activity

Mechanistic Insights into the Differential Catalysis by RheB and Its Mutants: Y35A and Y35A-D65A

RheB GTPase is a Ras-related molecular switch, which regulates the mTOR signaling pathway by cycling between the active [guanosine triphosphate (GTP)] state and inactive [guanine diphosphate (GDP)] state. Impairment of GTPase activity because of mutations in several small GTPases is known to be associated with several cancers. The conventional GTPase mechanism such as in H-Ras requires a conserved glutamine (Q64) in the switch-II region of RheB to align the catalytic water molecule for efficient GTP hydrolysis. The conformation of this conserved glutamine is different in RheB, resulting in an altered conformation of the entire switch-II region. Studies on the atypical switch-II conformation in RheB revealed a distinct, noncanonical mode of GTP hydrolysis. An RheB mutant Y35A was previously shown to exclusively enhance the intrinsic GTPase activity of RheB, whereas the Y35A-D65A double mutant was shown to reduce the elevated GTPase activity. Here, we have used all-atom molecular dynamics (MD) simulations for comprehensive understanding of the conformational dynamics associated with the fast (Y35A) and slow (Y35A-D65A) hydrolyzing mutants of RheB. Using a combination of starting models from PDB structures and in-silico generated mutant structures, we discuss the observed conformational deviations in wild type (WT) versus mutants. Our results show that a number of interactions of RheB with phosphates of GTP as well as Mg²⁺ are destabilized in Y35A mutant in the switch-I region. We report distinct water dynamics at the active site of WT and mutants. Furthermore, principal component analysis showed significant differences in the conformational space sampled by the WT and mutants. Our observations provide improved understanding of the noncanonical GTP hydrolysis mechanism adopted by RheB and its modulation by Y35A and Y35A-D65A mutants

Atypical switch-I Arginine plays a catalytic role in GTP hydrolysis by Rab21 from Entamoeba histolytica

Entamoeba histolytica, the causative agent of amoebic dysentery, liver abscess and colitis, exploits its vesicular trafficking machinery for survival and virulence. Rab family of small GTPases play a key role in the vesicular transport by undergoing the GTP/GDP cycle which is central to the biological processes. Amoebic genome encodes several atypical Rab GTPases which are unique due to absence of conserved sequence motif(s) or atypical residues in their catalytic site [Saito-Nakano et al., 2005]. Previously, EhRab21 has been reported to involve in amoebic invasion and migration [Emmanuel et al., 2015]. The conserved Glutamine of switch-II region is universally accepted to be crucial for GTP hydrolysis. Mutations that reduce the sidechain polarity of Glutamine render the protein GTPase activity deficient [Krengel et al., 1990]. Here, we report a catalytic role of atypical switch-I Arginine (R36) in intrinsic GTP hydrolysis catalysed by EhRab21. Unlike the GTPase activity deficient QL mutants, the GTPase activity of EhRab21Q64L was found to be marginally enhanced compared to the wild-type protein. Although EhRab21R36L mutant showed normal GTPase activity, the double mutant (R36L/Q64L) was found to be GTPase deficient. Thus, EhRab21 is a unique member of small GTPase family in which an atypical switch-I Arginine is capable of driving GTP hydrolysis independent of the conserved switch-II Glutamine

SIRT2 deacetylase regulates the activity of GSK3 isoforms independent of inhibitory phosphorylation

Glycogen synthase kinase 3 (GSK3) is a critical regulator of diverse cellular functions involved in the maintenance of structure and function. Enzymatic activity of GSK3 is inhibited by N-terminal serine phosphorylation. However, alternate post-translational mechanism(s) responsible for GSK3 inactivation are not characterized. Here, we report that GSK3 alpha and GSK3 beta are acetylated at Lys246 and Lys183, respectively. Molecular modeling and/or molecular dynamics simulations indicate that acetylation of GSK3 isoforms would hinder both the adenosine binding and prevent stable interactions of the negatively charged phosphates. We found that SIRT2 deacetylates GSK3 beta, and thus enhances its binding to ATP. Interestingly, the reduced activity of GSK3 beta is associated with lysine acetylation, but not with phosphorylation at Ser9 in hearts of SIRT2-deficient mice. Moreover, GSK3 is required for the anti-hypertrophic function of SIRT2 in cardiomyocytes. Overall, our study identified lysine acetylation as a novel post-translational modification regulating GSK3 activity

SIRT2 regulates oxidative stress-induced cell death through deacetylation of c-Jun NH2-terminal kinase

c-Jun NH2-terminal kinases (JNKs) are responsive to stress stimuli and their activation regulate key cellular functions, including cell survival, growth, differentiation and aging. Previous studies demonstrate that activation of JNK requires dual phosphorylation by the mitogen-activated protein kinase kinases. However, other post-translational mechanisms involved in regulating the activity of JNK have been poorly understood. In this work, we studied the functional significance of reversible lysine acetylation in regulating the kinase activity of JNK. We found that the acetyl transferase p300 binds to, acetylates and inhibits kinase activity of JNK. Using tandem mass spectrometry, molecular modelling and molecular dynamics simulations, we found that acetylation of JNK at Lys153 would hinder the stable interactions of the negatively charged phosphates and prevent the adenosine binding to JNK. Our screening for the deacetylases found SIRT2 as a deacetylase for JNK. Mechanistically, SIRT2-dependent deacetylation enhances ATP binding and enzymatic activity of JNK towards c-Jun. Furthermore, SIRT2-mediated deacetylation favours the phosphorylation of JNK by MKK4, an upstream kinase. Our results indicate that deacetylation of JNK by SIRT2 promotes oxidative stress-induced cell death. Conversely, SIRT2 inhibition attenuates H2O2-mediated cell death in HeLa cells. SIRT2-deficient (SIRT2-KO) mice exhibit increased acetylation of JNK, which is associated with markedly reduced catalytic activity of JNK in the liver. Interestingly, SIRT2-KO mice were resistant to acetaminophen-induced liver toxicity. SIRT2-KO mice show lower cell death, minimal degenerative changes, improved liver function and survival following acetaminophen treatment. Overall, our work identifies SIRT2-mediated deacetylation of JNK as a critical regulator of cell survival during oxidative stress