Human histone deacetylase 6 (HDAC6) is the major deacetylase responsible for removing the acetyl group from Lys40 of α-tubulin (αK40), which is located lumenally in polymerized microtubules. Here, we provide a detailed kinetic analysis of tubulin deacetylation and HDAC6/microtubule interactions using individual purified components. Our data unequivocally show that free tubulin dimers represent the preferred HDAC6 substrate, with a K M value of 0.23 µM and a deacetylation rate over 1,500-fold higher than that of assembled microtubules. We attribute the lower deacetylation rate of microtubules to both longitudinal and lateral lattice interactions within tubulin polymers. Using TIRF microscopy, we directly visualized stochastic binding of HDAC6 to assembled microtubules without any detectable preferential binding to microtubule tips. Likewise, indirect immunofluorescence microscopy revealed that microtubule deacetylation by HDAC6 is carried out stochastically along the whole microtubule length, rather than from the open extremities. Our data thus complement prior studies on tubulin acetylation and further strengthen the rationale for the correlation between tubulin acetylation and microtubule age.

Recent years witnessed rapid expansion of our knowledge about structural features of human glutamate carboxypeptidase II (GCPII). There are over thirty X-ray structures of human GCPII (and of its close ortholog GCPIII) publicly available at present. They include structures of ligand-free wild-type enzymes, complexes of wild-type GCPII/GCPIII with structurally diversified inhibitors as well as complexes of the GCPII(E424A) inactive mutant with several substrates. Combined structural data were instrumental for elucidating the catalytic mechanism of the enzyme. Furthermore the detailed knowledge of the GCPII architecture and protein-inhibitor interactions offers mechanistic insight into structure-activity relationship studies and can be exploited for the rational design of novel GCPII-specific compounds. This review presents a summary of structural information that has been gleaned since 2005, when the first GCPII structures were solved.

Histone deacetylase 6 (HDAC6) is a multidomain cytosolic hydrolase acting mostly on nonhistone protein substrates. Investigations of the substrate specificity of HDAC6 are confounded by the presence of 2 catalytically active deacetylase domains (DD1 and DD2). In this study, acetylome peptide microarrays and peptide libraries were used to map the substrate specificity of DD1 and DD2 of human HDAC6. The results show that DD1 is solely responsible for the deacetylation of substrates harboring the acetyllysine at their C terminus, whereas DD2 exclusively deacetylates peptides with an internal acetyllysine residue. Also, statistical analysis of the deacetylation data revealed amino acid preferences at individual positions flanking the acetyllysine, where glycine and arginine residues are favored at positions N‐terminal to the central acetyllysine; negatively charged glutamate is strongly disfavored throughout the sequence. Finally, the deacylation activity of HDAC6 was profiled by using a panel of acyl derivatives of the optimized peptide substrate and showed that HDAC6 acts as a proficient deformylase. Our data thus offer a detailed insight into the substrate preferences of the individual HDAC6 domains at the peptide level, and these findings can in turn help in elucidating the biologic roles of the enzyme and facilitate the development of new domain‐specific inhibitors as research tools or therapeutic agents.—Kutil, Z., Skultetyova, L., Rauh, D., Meleshin, M., Snajdr, I., Novakova, Z., Mikesova, J., Pavlicek, J., Hadzima, M., Baranova, P., Havlinova, B., Majer, P., Schutkowski, M., Barinka, C. The unraveling of substrate specificity of histone deacetylase 6 domains using acetylome peptide microarrays and peptide libraries. FASEB J. 33,4035–4045 (2019). http://www.fasebj.org

Arylalkylamine N-acetyltransferase (AANAT) catalyzes the N-acetylation of serotonin, the penultimate step in the synthesis of melatonin. Pineal AANAT activity increases at night in all vertebrates, resulting in increased melatonin production. This increases circulating levels of melatonin, thereby providing a hormonal signal of darkness. Kinetic and structural analysis of AANAT has determined that one element is floppy. This element, termed Loop 1, is one of three loops that comprise the arylalkylamine binding pocket. During the course of chordate evolution, Loop 1 acquired the tripeptide CPL, and the enzyme became highly active. Here we focused on the functional importance of the CPL tripeptide and found that activity was markedly reduced when it was absent. Moreover, increasing the local flexibility of this tripeptide region by P64G and P64A mutations had the counterintuitive effect of reducing activity and reducing the overall movement of Loop 1, as estimated from Langevin dynamics simulations. Binding studies indicate that these mutations increased the off-rate constant of a model substrate without altering the dissociation constant. The structural kink and local rigidity imposed by Pro-64 may enhance activity by favoring configurations of Loop 1 that facilitate catalysis and do not become immobilized by intramolecular interactions.

CD69 is the earliest leukocyte activation antigen playing a pivotal role in cellular signaling. Here, we show that a globular C-terminal domain of CD69 belonging to C-type lectins binds calcium through Asp 171, Glu 185, and Glu 187 with K(d) approximately 54 microM. Closure of the calcium-binding site results in a conformational shift of Thr 107 and Lys 172. Interestingly, structural changes in all of these amino acids lead to the formation of high-affinity binding sites for N-acetyl-D-glucosamine. Similarly, a structural change in Glu 185 and Glu 187 contributes to a high-affinity site for N-acetyl-D-galactosamine. Site-directed mutagenesis and molecular modeling allowed us to describe the structural details of binding sites for both carbohydrates. These studies explain the importance of calcium for recognition of carbohydrates by CD69 and provide an important paradigm for the role of weak interactions in the immune system.

Isoxazole is a five-membered heterocycle that is widely used in drug discovery endeavors. Here, we report the design, synthesis, and structural and biological characterization of SS-208, a novel HDAC6-selective inhibitor containing the isoxazole-3-hydroxamate moiety as a zinc-binding group as well as a hydrophobic linker. A crystal structure of the Danio rerio HDAC6/SS-208 complex reveals a bidentate coordination of the active-site zinc ion that differs from the preferred monodentate coordination observed for HDAC6 complexes with phenylhydroxamate-based inhibitors. While SS-208 has minimal effects on the viability of murine SM1 melanoma cells in vitro, it significantly reduced in vivo tumor growth in a murine SM1 syngeneic melanoma mouse model. These findings suggest that the antitumor activity of SS-208 is mainly mediated by immune-related antitumor activity as evidenced by the increased infiltration of CD8+ and NK+ T cells and the enhanced ratio of M1 and M2 macrophages in the tumor microenvironment.

Inhibition of glutamate carboxypeptidase II (GCPII) is effective in preclinical models of neurological disorders associated with excessive activation of glutamatergic systems. Here we report synthesis, structural characterization, and biological activity of new hydroxamic acid-based inhibitors with nanomolar affinity for human GCPII. Crystal structures of GCPII/hydroxamate complexes revealed an unprecedented binding mode in which the putative P1' glutarate occupies the spacious entrance funnel rather than the conserved glutamate-binding S1' pocket. This unique binding mode provides a mechanistic explanation for the structure-activity relationship data, most notably the lack of enantiospecificity and the tolerance for bulky/hydrophobic functions as substituents of a canonical glutarate moiety. The in vivo pharmacokinetics profile of one of the inhibitors will be presented along with analgesic efficacy data from the rat chronic constrictive injury model of neuropathic pain.

BackgroundThe arylalkylamine N-acetyltransferase (AANAT) family is divided into structurally distinct vertebrate and non-vertebrate groups. Expression of vertebrate AANATs is limited primarily to the pineal gland and retina, where it plays a role in controlling the circadian rhythm in melatonin synthesis. Based on the role melatonin plays in biological timing, AANAT has been given the moniker "the Timezyme". Non-vertebrate AANATs, which occur in fungi and protists, are thought to play a role in detoxification and are not known to be associated with a specific tissue.ResultsWe have found that the amphioxus genome contains seven AANATs, all having non-vertebrate type features. This and the absence of AANATs from the genomes of Hemichordates and Urochordates support the view that a major transition in the evolution of the AANATs may have occurred at the onset of vertebrate evolution. Analysis of the expression pattern of the two most structurally divergent AANATs in Branchiostoma lanceolatum (bl) revealed that they are expressed early in development and also in the adult at low levels throughout the body, possibly associated with the neural tube. Expression is clearly not exclusively associated with the proposed analogs of the pineal gland and retina. blAANAT activity is influenced by environmental lighting, but light/dark differences do not persist under constant light or constant dark conditions, indicating they are not circadian in nature. bfAANATα and bfAANATδ' have unusually alkaline (> 9.0) optimal pH, more than two pH units higher than that of vertebrate AANATs.ConclusionsThe substrate selectivity profiles of bfAANATα and δ' are relatively broad, including alkylamines, arylalkylamines and diamines, in contrast to vertebrate forms, which selectively acetylate serotonin and other arylalkylamines. Based on these features, it appears that amphioxus AANATs could play several roles, including detoxification and biogenic amine inactivation. The presence of seven AANATs in amphioxus genome supports the view that arylalkylamine and polyamine acetylation is important to the biology of this organism and that these genes evolved in response to specific pressures related to requirements for amine acetylation.