Role of the carboxy-termini of tubulin on its chaperone-like activity

Targeting the tubulin C-terminal tail by charged small molecules

The disordered tubulin C-terminal tail (CTT), which possesses a higher degree of heterogeneity, is the target for the interaction of many proteins and cellular components. Compared to the seven well-described binding sites of microtubule-targeting agents (MTAs) that localize on the globular tubulin core, tubulin CTT is far less explored. Therefore, tubulin CTT can be regarded as a novel site for the development of MTAs with distinct biochemical and cell biological properties. Here, we designed and synthesized linear and cyclic peptides containing multiple arginines (RRR), which are complementary to multiple acidic residues in tubulin CTT. Some of them showed moderate induction and promotion of tubulin polymerization. The most potent macrocyclic compound 1f was found to bind to tubulin CTT and thus exert its bioactivity. Such RRR containing compounds represent a starting point for the discovery of tubulin CTT-targeting agents with therapeutic potential.

The alphabet of intrinsic disorder: II. Various roles of glutamic acid in ordered and intrinsically disordered proteins

The ability of a protein to fold into unique functional state or to stay intrinsically disordered is encoded in its amino acid sequence. Both ordered and intrinsically disordered proteins (IDPs) are natural polypeptides that use the same arsenal of 20 proteinogenic amino acid residues as their major building blocks. The exceptional structural plasticity of IDPs, their capability to exist as heterogeneous structural ensembles and their wide array of important disorder-based biological functions that complements functional repertoire of ordered proteins are all rooted within the peculiar differential usage of these building blocks by ordered proteins and IDPs. In fact, some residues (so-called disorder-promoting residues) are noticeably more common in IDPs than in sequences of ordered proteins, which, in their turn, are enriched in several order-promoting residues. Furthermore, residues can be arranged according to their “disorder promoting potencies,” which are evaluated based on the relative abundances of various amino acids in ordered and disordered proteins. This review continues a series of publications on the roles of different amino acids in defining the phenomenon of protein intrinsic disorder and concerns glutamic acid, which is the second most disorder-promoting residue.

Microtubule ionic conduction and its implications for higher cognitive functions

The neuronal cytoskeleton has been hypothesized to play a role in higher cognitive functions including learning, memory and consciousness. Experimental evidence suggests that both microtubules and actin filaments act as biological electrical wires that can transmit and amplify electric signals via the flow of condensed ion clouds. The potential transmission of electrical signals via the cytoskeleton is of extreme importance to the electrical activity of neurons in general. In this regard, the unique structure, geometry and electrostatics of microtubules are discussed with the expected impact on their specific functions within the neuron. Electric circuit models of ionic flow along microtubules are discussed in the context of experimental data, and the specific importance of both the tubulin C-terminal tail regions, and the nano-pore openings lining the microtubule wall is elucidated. Overall, these recent results suggest that ions, condensed around the surface of the major filaments of the cytoskeleton, flow along and through microtubules in the presence of potential differences, thus acting as transmission lines propagating intracellular signals in a given cell. The significance of this conductance to the functioning of the electrically active neuron, and to higher cognitive function is also discussed.

Anti-mitotic activity of colchicine and the structural basis for its interaction with tubulin

In this review, an attempt has been made to throw light on the mechanism of action of colchicine and its different analogs as anti-cancer agents. Colchicine interacts with tubulin and perturbs the assembly dynamics of microtubules. Though its use has been limited because of its toxicity, colchicine can still be used as a lead compound for the generation of potent anti-cancer drugs. Colchicine binds to tubulin in a poorly reversible manner with high activation energy. The binding interaction is favored entropically. In contrast, binding of its simple analogs AC or DAAC is enthalpically favored and commences with comparatively low activation energy. Colchicine-tubulin interaction, which is normally pH dependent, has been found to be independent of pH in the presence of microtubule-associated proteins, salts or upon cleavage of carboxy termini of tubulin. Biphasic kinetics of colchicines-tubulin interaction has been explained in light of the variation in the residues around the drug-binding site on beta-tubulin. Using the crystal structure of the tubulin-DAMAcolchicine complex, a detailed discussion on the pharmacophore concept that explains the variation of affinity for different colchicine site inhibitors (CSI) has been discussed.

Subcellular localization suggests novel functions for prolyl endopeptidase in protein secretion

For a long time, prolyl endopeptidase (PEP) was believed to inactivate neuropeptides in the extracellular space. However, reports on the intracellular activity of PEP suggest additional, as yet unidentified, physiological functions for this enzyme. Here, we demonstrate using biochemical methods of subcellular fractionation, immunocytochemical double-labelling procedures and localization of PEP-enhanced green fluorescent protein fusion proteins that PEP is mainly localized to the perinuclear space, and is associated with the microtubulin cytoskeleton in human neuroblastoma and glioma cell lines. Disassembly of the microtubules by nocodazole treatment disrupts both the fibrillar tubulin and PEP labelling. Furthermore, in a two-hybrid screen, PEP was identified as binding partner of tubulin. These findings indicate novel functions for PEP in axonal transport and/or protein secretion. Indeed, a metabolic labelling approach revealed that both PEP inhibition and PEP antisense mRNA expression result in enhanced peptide/protein secretion from human U-343 glioma cells. Because disturbances in intracellular transport and protein secretion mechanisms are associated with a number of ageing-associated neurodegenerative diseases, cell-permeable PEP inhibitors may be useful for the application in a variety of related clinical conditions.

The B-ring substituent at C-7 of colchicine and the α-C-terminus of tubulin communicate through the “tail-body” interaction

The carboxy terminals of alphabeta-tubulins are flexible regions rich in acidic amino acid residues that play an inhibitory role in the polymerization of tubulin to microtubules. We have shown that the binding of colchicine and its B-ring analogs (with C-7 substituents) to tubulin are pH sensitive and have high activation energies. Under identical conditions, the binding of analogs without C-7 substituents is pH independent and has lower activation energy. Beta-C-terminus-truncated tubulin (alphabeta(s)) shows similar pH sensitivity and activation energy to native tubulin (alphabeta). Removal of the C-termini of both subunits of tubulin (alpha(s)beta(s)) or the binding of a basic peptide P2 to the negatively charged alpha-C-terminus of tubulin causes a colchicine-tubulin interaction independent of pH with a low activation energy. Tubulin dimer structure shows that the C-terminal alpha-tail is too far from the colchicine binding site to interact directly with the bound colchicine. Therefore, it is likely that the interaction of the alpha-C-terminus with the main body of tubulin indirectly affects the colchicine-tubulin interaction via conformational changes in the main body. We therefore conclude that in the presence of tail-body interaction, a B-ring substituent makes contact with the alpha-tubulin and induces significant conformational changes in alpha-tubulin.

MAP2 prevents protein aggregation and facilitates reactivation of unfolded enzymes. Implications for the chaperone-like activity of MAP2

It is well established that in addition to its functional role in cell motility, cell division and intracellular transport, cytoskeletal protein tubulin also possesses significant chaperone-like activity. In vitro studies from our laboratory showed that dimeric tubulin can prevent stress induced aggregation of substrate proteins, can resist thermal deactivation of enzymes and can also refold enzymes from their fully denatured state [Manna, T., Sarkar, T., Poddar, A., Roychowdhury, M., Das, K.P. & Bhattacharyya, B. (2001) J. Biol. Chem.276, 39742-39747]. Negative charges of the C-termini of both subunits of tubulin are essential for this chaperone-like property as the deletion of only beta-C-terminus or the binding of a 14-residue basic peptide P2 to the alpha-C-terminus completely abolishes this property [Sarkar, T., Manna, T., Bhattacharyya, S., Mahapatra, P., Poddar, A., Roy, S., Pena, J., Solana, R., Tarazona, R. & Bhattacharyya, B. (2001) Proteins Struct. Funct. Genet.44, 262-269]. Based on these results, one would expect that the microtubular proteins (MTP, tubulin with microtubular-associated proteins, i.e. MAPs bound to the C-terminus) should not possess any chaperone-like activity. To our surprise we noticed excellent chaperone-like activity of MTP. MTP prevents chemical and thermal aggregation of other proteins and can enhance the extent of refolding of fully unfolded substrate enzymes. Because MTP contains tubulin as well as several MAPs bound to the C-termini of tubulin, we fractionated and purified microtubular associated protein 2 (MAP2) and tau using phosphocellulose chromatography. Experiments with purified proteins demonstrated that it is the MAP2 of MTP that exhibits significant chaperone-like activity. This has been shown by the prevention of dithiothreitol-induced aggregation of insulin, thermal aggregation of alcohol dehydrogenase and regain of enzymatic activity during refolding of unfolded substrates. Tau, which shares a homologous C-terminal domain with MAP2, possesses no such activity.

A small heat shock/α-crystallin protein from encysted Artemia embryos suppresses tubulin denaturation

Small heat shock/alpha-crystallin proteins function as molecular chaperones, protecting other proteins from irreversible denaturation by an energy-independent process. The brine shrimp, Artemia franciscana, produces a small heat shock/alpha-crystallin protein termed p26, found in embryos undergoing encystment, diapause, and metabolic arrest. These embryos withstand long-term anoxia and other stresses normally expected to cause death, a property likely dependent on molecular chaperone activity. The association of p26 with tubulin in unfractionated cell-free extracts of Artemia embryos was established by affinity chromatography, suggesting that p26 chaperones tubulin during encystment. To test this possibility, both proteins were purified by modifying published protocols, thereby simplifying the procedures, enhancing p26 yield about 2-fold, and recovering less tubulin than before. The denaturation of purified tubulin as it "aged" and exposed hydrophobic sites during incubation at 35 degrees C was greatly reduced when p26 was present; however, tubulin polymerization into microtubules was reduced. On incubation at 35 degrees C, centrifugation in sucrose density gradients demonstrated the association of purified p26 with tubulin. This is the first study where the relationship between a small heat shock/alpha-crystallin protein and tubulin from the same physiologically stressed organism was examined. The results support the proposal that p26 binds tubulin and prevents its denaturation, thereby increasing the resistance of encysted Artemia embryos to stress. Additional factors are apparently required for release of tubulin from p26 and restoration of efficient assembly, events that would occur as embryos resume development and the need for microtubules is established.