Srinath Krishnamurthy

Cyclic AMP- and (Rp)-cAMPS-induced Conformational Changes in a Complex of the Catalytic and Regulatory (RIα) Subunits of Cyclic AMP-dependent Protein Kinase

We took a discovery approach to explore the actions of cAMP and two of its analogs, one a cAMP mimic ((Sp)-adenosine cyclic 3′:5′-monophosphorothioate ((Sp)-cAMPS)) and the other a diastereoisomeric antagonist ((Rp)-cAMPS), on a model system of the type Iα cyclic AMP-dependent protein kinase holoenzyme, RIα(91–244)·C-subunit, by using fluorescence spectroscopy and amide H/2H exchange mass spectrometry. Specifically, for the fluorescence experiments, fluorescein maleimide was conjugated to three cysteine single residue substitution mutants, R92C, T104C, and R239C, of RIα(91–244), and the effects of cAMP, (Sp)-cAMPS, and (Rp)-cAMPS on the kinetics of R-C binding and the time-resolved anisotropy of the reporter group at each conjugation site were measured. For the amide exchange experiments, ESI-TOF mass spectrometry with pepsin proteolytic fragmentation was used to assess the effects of (Rp)-cAMPS on amide exchange of the RIα(91–244)·C-subunit complex. We found that cAMP and its mimic perturbed at least parts of the C-subunit interaction Sites 2 and 3 but probably not Site 1 via reduced interactions of the linker region and αC of RIα(91–244). Surprisingly, (Rp)-cAMPS not only increased the affinity of RIα(91–244) toward the C-subunit by 5-fold but also produced long range effects that propagated through both the C- and R-subunits to produce limited unfolding and/or enhanced conformational flexibility. This combination of effects is consistent with (Rp)-cAMPS acting by enhancing the internal entropy of the R·C complex. Finally, the (Rp)-cAMPS-induced increase in affinity of RIα(91–244) toward the C-subunit indicates that (Rp)-cAMPS is better described as an inverse agonist because it decreases the fractional dissociation of the cyclic AMP-dependent protein kinase holoenzyme and in turn its basal activity.

Architecture of a dsDNA Viral Capsid in Complex with Its Maturation Protease

Most double-stranded DNA (dsDNA) viruses, including bacteriophages and herpesviruses, rely on a staged assembly process of capsid formation. A viral protease is required for many of them to disconnect scaffolding domains/proteins from the capsid shell, therefore priming the maturation process. We used the bacteriophage HK97 as a model system to decipher the molecular mechanisms underlying the recruitment of the maturation protease by the assembling procapsid and the influence exerted onto the latter. Comparisons of the procapsid with and without protease using single-particle cryoelectron microscopy reconstructions, hydrogen/deuterium exchange coupled to mass spectrometry, and native mass spectrometry demonstrated that the protease interacts with the scaffolding domains within the procapsid interior and stabilizes them as well as the whole particle. The results suggest that the thermodynamic consequences of protease packaging are to shift the equilibrium between isolated coat subunit capsomers and procapsid in favor of the latter by stabilizing the assembled particle before making the process irreversible through proteolysis of the scaffolding domains.

Predicting allosteric effects from orthosteric binding in Hsp90-ligand interactions : implications for fragment-based drug design

A key question in mapping dynamics of protein-ligand interactions is to distinguish changes at binding sites from those associated with long range conformational changes upon binding at distal sites. This assumes a greater challenge when considering the interactions of low affinity ligands (dissociation constants, KD, in the μM range or lower). Amide hydrogen deuterium Exchange mass spectrometry (HDXMS) is a robust method that can provide both structural insights and dynamics information on both high affinity and transient protein-ligand interactions. In this study, an application of HDXMS for probing the dynamics of low affinity ligands to proteins is described using the N-terminal ATPase domain of Hsp90. Comparison of Hsp90 dynamics between high affinity natural inhibitors (KD ~ nM) and fragment compounds reveal that HDXMS is highly sensitive in mapping the interactions of both high and low affinity ligands. HDXMS reports on changes that reflect both orthosteric effects and allosteric changes accompanying binding. Orthosteric sites can be identified by overlaying HDXMS onto structural information of protein-ligand complexes. Regions distal to orthosteric sites indicate long range conformational changes with implications for allostery. HDXMS, thus finds powerful utility as a high throughput method for compound library screening to identify binding sites and describe allostery with important implications for fragment-based ligand discovery (FBLD).

Dynamics of phosphodiesterase-induced cAMP dissociation from protein kinase A: Capturing transient ternary complexes by HDXMS

cAMP signaling is a fundamental cellular process necessary for mediating responses to hormonal stimuli. In contrast to cAMP-dependent activation of protein kinase A (PKA), an important cellular target, far less is known on termination in cAMP signaling, specifically how phosphodiesterases (PDEs) facilitate dissociation and hydrolysis of bound cAMP. In this study, we have probed the dynamics of a ternary complex of PKA and a PDE-RegA with an excess of a PDE-nonhydrolyzable cAMP analog, Sp-cAMPS by amide hydrogen/deuterium exchange mass spectrometry (HDXMS). Our results highlight how HDXMS can be used to monitor reactions together with mapping conformational dynamics of transient signaling complexes. Our results confirm a two-state model for active RegA-mediated dissociation of bound cAMP. Further, our results reveal that Sp-cAMPS and RegA mediate mutually exclusive interactions with the same region of PKA and at specific concentrations of Sp-cAMPS, RegA is capable of blocking Sp-cAMPS reassociation to PKA. This provides a molecular basis for how PDEs modulate levels of intracellular cAMP so that PKA is better suited to responding to fluxes rather than constant levels of cAMP. This study underscores how HDXMS can be a powerful tool for monitoring reactions together with mapping conformational dynamics in signaling proteins. This article is part of a Special Issue entitled: Mass spectrometry in structural biology.

β-subunit myristoylation functions as an energy sensor by modulating the dynamics of AMP-activated Protein Kinase

The heterotrimeric AMP-activated protein kinase (AMPK), consisting of α, β and γ subunits, is a stress-sensing enzyme that is activated by phosphorylation of its activation loop in response to increases in cellular AMP. N-terminal myristoylation of the β-subunit has been shown to suppress Thr172 phosphorylation, keeping AMPK in an inactive state. Here we use amide hydrogen-deuterium exchange mass spectrometry (HDX-MS) to investigate the structural and dynamic properties of the mammalian myristoylated and non-myristoylated inactivated AMPK (D139A) in the presence and absence of nucleotides. HDX MS data suggests that the myristoyl group binds near the first helix of the C-terminal lobe of the kinase domain similar to other kinases. Our data, however, also shows that ATP.Mg2+ results in a global stabilization of myristoylated, but not non-myristoylated AMPK, and most notably for peptides of the activation loop of the α-kinase domain, the autoinhibitory sequence (AIS) and the βCBM. AMP does not have that effect and HDX measurements for myristoylated and non-myristoylated AMPK in the presence of AMP are similar. These differences in dynamics may account for a reduced basal rate of phosphorylation of Thr172 in myristoylated AMPK in skeletal muscle where endogenous ATP concentrations are very high.

Distinguishing direct binding interactions from allosteric effects in the protease-HK97 prohead I δ domain complex by amide H/D exchange mass spectrometry.

A major question in mapping protein-ligand or protein-protein interactions in solution is to distinguish direct-binding interactions from long-range conformational changes at allosteric sites. We describe here the applicability of amide hydrogen deuterium exchange mass spectrometry (HDXMS) in addressing this important question using the bacteriophage HK97 capsid proteins’ interactions with their processing protease. HK97 is a lambda-like dsDNA bacteriophage that is ideal for studies of particle assembly and maturation. Its capsid precursor protein is composed of two main regions, the scaffolding protein (δ-domain, residues 2-103), and the coat subunit (residues 104-385), which spontaneously forms a mixture of hexamers and pentamers upon association. Activation of the viral protease, which occurs after particle assembly, is initiated by the protease mediated digestion of the scaffolding domains to yield Prohead-2. This irreversible step is obligatory for activation of the virus maturation pathway. Here we provide an “addendum” to our previous study of Prohead I and Prohead I+pro (a transient complex of Prohead I and the protease) where we investigated the interactions between the δ domain and the packaged protease using HDXMS. Our results revealed two sites on the δ domain: one set of contiguous peptides that showed decreased exchange at the protease binding site at early time points of deuterium labeling and another separate set of continuous peptides that showed decreased exchange at later time points. Even though this cannot reveal the time scales of molecular processes governing binding and allostery, we believe this differential pattern of exchange across deuteration times can allow spatial distinction between binding sites and long range conformational changes with allosteric implications. This partitioning can be discerned from the lag between noncontiguous regions on a protein showing maximal changes in deuterium exchange and highlights a powerful application of HDXMS in distinguishing direct binding in transient protein-protein interactions from allosteric changes.