Zn²+-induced rearrangement of the disordered TPPP/p25 affects its microtubule assembly and GTPase activity

Perspective Strategies for Interventions in Parkinsonism: Remedying the Neglected Role of TPPP

Neurological disorders such as Parkinsonism cause serious socio-economic problems as there are, at present, only therapies that treat their symptoms. The well-established hallmark alpha-synuclein (SYN) is enriched in the inclusion bodies characteristic of Parkinsonism. We discovered a prominent partner of SYN, termed Tubulin Polymerization Promoting Protein (TPPP), which has important physiological and pathological activities such as the regulation of the microtubule network and the promotion of SYN aggregation. The role of TPPP in Parkinsonism is often neglected in research, which we here attempt to remedy. In the normal brain, SYN and TPPP are expressed endogenously in neurons and oligodendrocytes, respectively, whilst, at an early stage of Parkinsonism, soluble hetero-associations of these proteins are found in both cell types. The cell-to-cell transmission of these proteins, which is central to disease progression, provides a unique situation for specific drug targeting. Different strategies for intervention and for the discovery of biomarkers include (i) interface targeting of the SYN-TPPP hetero-complex; (ii) proteolytic degradation of SYN and/or TPPP using the PROTAC technology; and (iii) depletion of the proteins by miRNA technology. We also discuss the potential roles of SYN and TPPP in the phenotype stabilization of neurons and oligodendrocytes.

Dynamin-like Protein 1 (DNML1) as a Molecular Target for Antibody-Based Immunotherapy to Treat Glaucoma

Slow and progressive loss of retinal ganglion cells (RGCs) is the main characteristic of glaucoma, the second leading cause of blindness worldwide. Previous studies have shown that impaired mitochondrial dynamics could facilitate retinal neurodegeneration. Mitochondrial dynamics are regulated directly (fission) or more indirectly (fusion) by dynamin-like protein 1 (DNML1). Therefore, DNM1L might be a promising target for an antibody-based approach to treat glaucoma. The consequences of targeting endogenous DNM1L by antibodies in a glaucoma animal model have not been investigated yet. Here, we show that the intravitreal application of an anti-DNM1L antibody showed protective effects regarding the survival of RGCs and their axons in the retinal nerve fiber layer (RNFL). Antibody treatment also improved retinal functionality, as observed by electroretinography (Ganzfeld ERG). Western blot analysis revealed altered DNM1L phosphorylation and altered expression of proteins related to apoptosis suggesting a decreased apoptosis rate. Mass spectrometry analysis revealed 28 up-regulated and 21 down-regulated proteins (p < 0.05) in both experimental groups. Protein pathway analysis showed that many proteins interacted directly with the target protein DNM1L and could be classified into three main protein clusters: Vesicle traffic-associated (NSF, SNCA, ARF1), mitochondrion-associated (HSP9A, SLC25A5/ANT2, GLUD1) and cytoskeleton-associated (MAP1A) signaling pathway. Our results demonstrate that DNM1L is a promising target for an antibody-based approach to glaucoma therapy.

Modulatory Role of TPPP3 in Microtubule Organization and Its Impact on Alpha-Synuclein Pathology

Parkinson’s disease is characterized by locomotion deficits, dopaminergic neuronal loss and alpha-synuclein (SYN) aggregates; the Tubulin Polymerization Promoting Protein (TPPP/p25 or TPPP1) is also implicated in these processes. The moonlighting and chameleon TPPP1 modulates the dynamics/stability of the multifunctional microtubule network by promoting its acetylation and bundling. Previously, we identified the microtubule-associated TPPP3, a homologue of TPPP1 lacking its N-terminus; however, its involvement in physiological or pathological processes was not elucidated. In this work, we have shown the modulatory role of TPPP3, similarly to TPPP1, in microtubule organization, as well as its homo- and hetero-associations with TPPP1. TPPP3, in contrast to TPPP1, virtually does not bind to SYN; consequently, it does not promote SYN aggregation. Its anti-aggregative potency is achieved by counteracting the formation of the TPPP1–SYN pathological complex/aggregation leading to Parkinsonism. The interactions of TPPP3 have been determined and quantified in vitro with recombinant human proteins, cell extracts and in living human cells using different methods including bifunctional fluorescence complementation. The tight association of TPPP3 with TPPP1, but not with SYN, may ensure a unique mechanism for its inhibitory effect. TPPP3 or its selected fragments may become a leading agent for developing anti-Parkinson agents.

Tubulin Polymerization Promoting Protein, Ringmaker, and MAP1B Homolog Futsch Coordinate Microtubule Organization and Synaptic Growth

Drosophila Ringmaker (Ringer) is homologous to the human Tubulin Polymerization Promoting Proteins (TPPPs) that are implicated in the stabilization and bundling of microtubules (MTs) that are particularly important for neurons and are also implicated in synaptic organization and plasticity. No in vivo functional data exist that have addressed the role of TPPP in synapse organization in any system. Here, we present the phenotypic and functional characterization of ringer mutants during Drosophila larval neuromuscular junction (NMJ) synaptic development. ringer mutants show reduced synaptic growth and transmission and display phenotypic similarities and genetic interactions with the Drosophila homolog of vertebrate Microtubule Associated Protein (MAP)1B, futsch. Immunohistochemical and biochemical analyses show that individual and combined loss of Ringer and Futsch cause a significant reduction in MT loops at the NMJs and reduced acetylated-tubulin levels. Presynaptic over-expression of Ringer and Futsch causes elevated levels of acetylated-tubulin and significant increase in NMJ MT loops. These results indicate that Ringer and Futsch regulate synaptic MT organization in addition to synaptic growth. Together our findings may inform studies on the close mammalian homolog, TPPP, and provide insights into the role of MTs and associated proteins in synapse growth and organization.

Localization of the zinc binding tubulin polymerization promoting protein in the mice and human eye

Tubulin Polymerization Promoting Protein (TPPP/p25) modulates the dynamics and stability of the microtubule network by its bundling and acetylation enhancing activities that can be modulated by the binding of zinc to TPPP/p25. Its expression is essential for the differentiation of oligodendrocytes, the major constituents of the myelin sheath, and has been associated with neuronal inclusions. In this paper, evidence is provided for the expression and localization of TPPP/p25 in the zinc-rich retina and in the oligodendrocytes in the optic nerve. Localization of TPPP/p25 was established by confocal microscopy using calbindin and synaptophysin as markers of specific striations in the inner plexiform layer (IPL) and presynaptic terminals, respectively. Postsynaptic nerve terminals in striations S1, S3 and S5 in the IPL and a subset of amacrine cells show immunopositivity against TPPP/p25 both in mice and human eyes. The co-localization of TPPP/p25 with acetylated tubulin was detected in amacrine cells, oligodendrocyte cell bodies and in synapses in the IPL. Quantitative Western blot revealed that the TPPP/p25 level in the retina was 0.05-0.13 ng/μg protein, comparable to that in the brain. There was a central (from optic nerve head) to peripheral retinal gradient in TPPP/p25 protein levels. Our in vivo studies revealed that the oral zinc supplementation of mice significantly increased TPPP/p25 as well as acetylated tubulin levels in the IPL. These results suggest that TPPP/p25, a microtubule stabilizer can play a role in the organization and reorganization of synaptic connections and visual integration in the eye.

Modulation Of Microtubule Acetylation By The Interplay Of TPPP/p25, SIRT2 And New Anticancer Agents With Anti-SIRT2 Potency

The microtubule network exerts multifarious functions controlled by its decoration with various proteins and post-translational modifications. The disordered microtubule associated Tubulin Polymerization Promoting Protein (TPPP/p25) and the NAD⁺-dependent tubulin deacetylase sirtuin-2 (SIRT2) play key roles in oligodendrocyte differentiation by acting as dominant factors in the organization of myelin proteome. Herein, we show that SIRT2 impedes the TPPP/p25-promoted microtubule assembly independently of NAD⁺; however, the TPPP/p25-assembled tubulin ultrastructures were resistant against SIRT2 activity. TPPP/p25 counteracts the SIRT2-derived tubulin deacetylation producing enhanced microtubule acetylation. The inhibition of the SIRT2 deacetylase activity by TPPP/p25 is evolved by the assembly of these tubulin binding proteins into a ternary complex, the concentration-dependent formation of which was quantified by experimental-based mathematical modelling. Co-localization of the SIRT2-TPPP/p25 complex on the microtubule network was visualized in HeLa cells by immunofluorescence microscopy using Bimolecular Fluorescence Complementation. We also revealed that a new potent SIRT2 inhibitor (MZ242) and its proteolysis targeting chimera (SH1) acting together with TPPP/p25 provoke microtubule hyperacetylation, which is coupled with process elongation only in the case of the degrader SH1. Both the structural and the functional effects manifesting themselves by this deacetylase proteome could lead to the fine-tuning of the regulation of microtubule dynamics and stability.

Controlled Zn2+-Triggered Drug Release by Preferred Coordination of Open Active Sites within Functionalization Indium Metal Organic Frameworks

Drug delivery in target regions could make extraordinary progress in chemoselective therapies. A novel preferred coordination (PC) strategy referring to proactive interacting with open active sites to replace previous occupation by ion-exchange for controlling release of drug molecules is well-constructed. Two topological types of MOF-In1 (Schläfli symbol: (4,8)-connected of (4¹⁰·6¹⁵·8³)(4⁵·6)₂) and MOF-In2 (Schläfli symbol: (4,4)-connected of (6⁶)) show the specific way. Increasing node connectivity as well as the trapping of guest OH^- anions, 5-fluorouracil (5-FU) is preferentially captured into the MOF-In1, which exhibits an outstanding loading capacity around 34.32 wt %. ¹⁹F NMR spectroscopy was further employed to investigate host-guest interaction and reveal the binding constant (K_a = 3.84 × 10² M^-1). Meanwhile, the controlled release of 5-FU in a simulated human body with liquid phosphate-buffered saline solution by biofriendly Zn²⁺-triggered is realized. With an elevated Zn²⁺ concentration, the drug release will be enhanced. This efficient strategy for MOFs as multifunctional drug carrier opens a new avenue for biological and medical applications.

Challenging drug target for Parkinson's disease: Pathological complex of the chameleon TPPP/p25 and alpha-synuclein proteins

The hallmarks of Parkinson's disease and other synucleinopathies, Tubulin Polymerization Promoting Protein (TPPP/p25) and α-synuclein (SYN) have two key features: they are disordered and co-enriched/co-localized in brain inclusions. These Neomorphic Moonlighting Proteins display both physiological and pathological functions due to their interactions with distinct partners. To achieve the selective targeting of the pathological TPPP/p25-SYN but not the physiological TPPP/p25-tubulin complex, their interfaces were identified as a specific innovative strategy for the development of anti-Parkinson drugs. Therefore, the interactions of TPPP/p25 with tubulin and SYN were characterized which suggested the involvements of the 178-187 aa and 147-156 aa segments in the complexation of TPPP/p25 with tubulin and SYN, respectively. However, various truncated and deletion mutants reduced but did not abolish the interactions except one mutant; in addition synthetized fragments corresponding to the potential binding segments of TPPP/p25 failed to interact with SYN. In fact, the studies of the multiple interactions at molecular and cellular levels revealed the high conformational plasticity, chameleon feature, of TPPP/p25 that ensures exceptional functional resilience; the lack of previously identified binding segments could be replaced by other segments. The experimental results are underlined by distinct bioinformatics tools. All these data revealed that although targeting chameleon proteins is a challenging task, nevertheless, the validation of a drug target can be achieved by identifying the interface of complexes of the partner proteins existing at the given pathological conditions.

Differential effects of zinc binding on structured and disordered regions in the multidomain STIL protein

Here we show that simultaneous binding of Zn²⁺ ions has different effects on structured and disordered domains in the same multidomain protein.

Binding of metal ions is an important regulatory mechanism in proteins. Specifically, Zn²⁺ binding to disordered regions commonly induces a disorder to order transition and gain of structure or oligomerization. Here we show that simultaneous binding of Zn²⁺ ions has different effects on structured and disordered domains in the same multidomain protein. The centrosomal STIL protein bound Zn²⁺ ions via both its structured N-terminal domain (NTD) and disordered central region (IDR). Zn²⁺ binding induced structural rearrangement of the structured NTD but promoted oligomerization of the IDR. We suggest that by binding Zn²⁺ STIL acquires a different conformation, which allows its oligomerization and induces its activity. Sequence alignment of the oligomerization region revealed a new suggested motif, SxKxS/SxHxS/SxLxS, which may participate in STIL oligomerization. Binding of the same metal ion through a disordered and a structured domain in the same protein is a property that may have implications in regulating the protein activity. By doing so, the protein achieves two parallel outcomes: structural changes and oligomerization that can take place together. Our results describe a new important role of the delicate interplay between structure and intrinsic disorder in proteins.

Targeting the interface of the pathological complex of α-synuclein and TPPP/p25

The pathological interaction of intrinsically disordered proteins, such as α-synuclein (SYN) and Tubulin Polymerization Promoting Protein (TPPP/p25), is often associated with neurodegenerative disorders. These hallmark proteins are co-enriched and co-localized in brain inclusions of Parkinson's disease and other synucleinopathies; yet, their successful targeting does not provide adequate effect due to their multiple functions. Here we characterized the interactions of the human recombinant wild type SYN, its truncated forms (SYN(1-120), SYN(95-140)), a synthetized peptide (SYN(126-140)) and a proteolytic fragment (SYN(103-140)) with TPPP/p25 to identify the SYN segment involved in the interaction. The binding of SYN(103-140) to TPPP/p25 detected by ELISA suggested the involvement of a segment within the C-terminal of SYN. The studies performed with ELISA, Microscale Thermophoresis and affinity chromatography proved that SYN(95-140) and SYN(126-140) - in contrast to SYN(1-120) - displayed significant binding to TPPP/p25. Fluorescence assay with ANS, a molten globule indicator, showed that SYN, but not SYN(1-120) abolished the zinc-induced local folding of both the full length as well as the N- and C-terminal-free (core) TPPP/p25; SYN(95-140) and SYN(126-140) were effective as well. The aggregation-prone properties of the SYN species with full length or core TPPP/p25 visualized by immunofluorescent microscopy demonstrated that SYN(95-140) and SYN(126-140), but not SYN(1-120), induced co-enrichment and massive intracellular aggregation after their premixing and uptake from the medium. These data with their innovative impact could contribute to the development of anti-Parkinson drugs with unique specificity by targeting the interface of the pathological TPPP/p25-SYN complex.

Zn(2+)-Triggered Drug Release from Biocompatible Zirconium MOFs Equipped with Supramolecular Gates

A new theranostic nanoplatform, comprising of monodisperse zirconium metal-organic frameworks (MOFs) as drug carriers and carboxylatopillar[5]arene-based supramolecular switches as gating entities, is constructed, and controlled drug release triggered by bio-friendly Zn(2+) ions (abundant in synaptic vesicles) and auxiliary thermal stimulus is realized. This on-command drug delivery system exhibits large pore sizes for drug encapsulation, excellent biodegradability and biocompatibility, extremely low cytotoxicity and premature drug release, and superior dual-stimuli responsiveness, opening a new avenue in targeted drug delivery and controlled release of therapeutic agents, especially in the treatment of central nervous system diseases.

Dual life of TPPP/p25 evolved in physiological and pathological conditions

Neomorphic moonlighting proteins perform distinct functions under physiological and pathological conditions without alterations at the gene level. The disordered tubulin-polymerization-promoting protein (TPPP/p25), a prototype of neomorphic moonlighting proteins, modulates the dynamics and stability of the microtubule system via its bundling and tubulin acetylation-promoting activities. These physiological functions are mediated by its direct associations with tubulin/microtubules as well as tubulin deacetylases such as histone deacetylase (HDAC) 6. In a normal brain, TPPP/p25 is expressed in oligodendrocytes and plays a crucial role in the formation of projections in the course of differentiation required for axon ensheathment. Under pathological conditions, TPPP/p25 interacts with α-synuclein, an aberrant protein-protein interaction resulting in aggregation leading to the formation of inclusions as clinical symptoms. The co-enrichment and co-localization of TPPP/p25 and α-synuclein were established in human-brain inclusions characteristic of Parkinson's disease (PD) and other synucleinopathies. The binding segments on TPPP/p25 involved in the physiological and the pathological interactions were identified and validated at molecular and cellular levels using recombinant proteins and transfected HeLa and inducible Chinese-hamster ovary (CHO) 10 cells expressing TPPP/p25. Our finding that distinct motifs are responsible for the neomorphic moonlighting feature of TPPP/p25, has powerful innovative effects in anti-Parkinson's disease drug research.

Stimuli-responsive metal-organic frameworks gated by pillar[5]arene supramolecular switches

Spurred on by recent advances in materials chemistry and drug delivery, a new stimuli-responsive theranostic hybrid platform, based on mechanized monodisperse nano metal-organic frameworks (NMOFs) gated by carboxylatopillar[5]arene (CP5) switches with bio-friendly pH-triggered cargo release capabilities, has been constructed for the first time. This nanoscale smart cargo delivery system showed pH- and/or competitive binding agent-triggered controlled cargo release with negligible premature release, large pore sizes for drug encapsulation, low cytotoxicity, good biodegradability and biocompatibility, and potential application in cell imaging, which offers a new tool in targeted drug delivery and the controlled release of therapeutic agents.

Zinc-induced structural changes of the disordered tppp/p25 inhibits its degradation by the proteasome

Tubulin Polymerization Promoting Protein/p25 (TPPP/p25), a neomorphic moonlighting protein displaying both physiological and pathological functions, plays a crucial role in the differentiation of the zinc-rich oligodendrocytes, the major constituent of myelin sheath; and it is enriched and co-localizes with α-synuclein in brain inclusions hallmarking Parkinson's disease and other synucleinopathies. In this work we showed that the binding of Zn(2+) to TPPP/p25 promotes its dimerization resulting in increased tubulin polymerization promoting activity. We also demonstrated that the Zn(2+) increases the intracellular TPPP/p25 level resulting in a more decorated microtubule network in CHO10 and CG-4 cells expressing TPPP/p25 ectopically and endogenously, respectively. This stabilization effect is crucial for the differentiation and aggresome formation under physiological and pathological conditions, respectively. The Zn(2+)-mediated effect was similar to that produced by treatment of the cells with MG132, a proteasome inhibitor or Zn(2+) plus MG132 as quantified by cellular ELISA. The enhancing effect of zinc ion on the level of TPPP/p25 was independent of the expression level of the protein produced by doxycycline induction at different levels or inhibition of the protein synthesis by cycloheximide. Thus, we suggest that the zinc as a specific divalent cation could be involved in the fine-tuning of the physiological TPPP/p25 level counteracting both the enrichment and the lack of this protein leading to distinct central nervous system diseases.

Relocation of p25α/tubulin polymerization promoting protein from the nucleus to the perinuclear cytoplasm in the oligodendroglia of sporadic and COQ2 mutant multiple system atrophy

p25α/tubulin polymerization promoting protein (TPPP) is an oligodendroglial protein that plays crucial roles including myelination, and the stabilization of microtubules. In multiple system atrophy (MSA), TPPP is suggested to relocate from the myelin sheath to the oligodendroglial cell body, before the formation of glial cytoplasmic inclusions (GCIs), the pathologic hallmark of MSA. However, much is left unknown about the re-distribution of TPPP in MSA. We generated new antibodies against the N- and C-terminus of TPPP, and analyzed control and MSA brains, including the brain of a familial MSA patient carrying homozygous mutations in the coenzyme Q2 gene (COQ2). In control brain tissues, TPPP was localized not only in the cytoplasmic component of the oligodendroglia including perinuclear cytoplasm and peripheral processes in the white matter, but also in the nucleus of a fraction (62.4%) of oligodendroglial cells. Immunoelectron microscopic analysis showed TPPP in the nucleus and mitochondrial membrane of normal oligodendroglia, while western blot also supported its nuclear and mitochondrial existence. In MSA, the prevalence of nuclear TPPP was 48.6% in the oligodendroglia lacking GCIs, whereas it was further decreased to 19.6% in the oligodendroglia with phosphorylated α-synuclein (pα-syn)-positive GCIs, both showing a significant decrease compared to controls (62.4%). In contrast, TPPP accumulated in the perinuclear cytoplasm where mitochondrial membrane (TOM20 and cytochrome C) and fission (DRP1) proteins were often immunoreactive. We conclude that in MSA-oligodendroglia, TPPP is reduced, not only in the peripheral cytoplasm, but also in the nucleus and relocated to the perinuclear cytoplasm.

Identification of motives mediating alternative functions of the neomorphic moonlighting TPPP/p25

The disordered Tubulin Polymerization Promoting Protein (TPPP/p25), a prototype of neomorphic moonlighting proteins, displays physiological and pathological functions by interacting with distinct partners. Here the role of the disordered N- and C-termini straddling a middle flexible segment in the distinct functions of TPPP/p25 was established, and the binding motives responsible for its heteroassociations with tubulin and α-synuclein, its physiological and pathological interacting partner, respectively, were identified. We showed that the truncation of the disordered termini altered the folding state of the middle segment and has functional consequences concerning its physiological function. Double truncation diminished its binding to tubulin/microtubules, consequently the tubulin polymerization/microtubule bundling activities of TPPP/p25 were lost highlighting the role of the disordered termini in its physiological function. In contrast, interaction of TPPP/p25 with α-synuclein was not affected by the truncations and its α-synuclein aggregation promoting activity was preserved, showing that the α-synuclein binding motif is localized within the middle segment. The distinct tubulin and α-synuclein binding motives of TPPP/p25 were also demonstrated at the cellular level: the double truncated TPPP/p25 did not align along the microtubules in contrast to the full length form, while it induced α-synuclein aggregation. The localization of the binding motives on TPPP/p25 were established by specific ELISA experiments performed with designed and synthesized peptides: motives at the 178-187 and 147-156 segments are involved in the binding of tubulin and α-synuclein, respectively. The dissimilarity of these binding motives responsible for the neomorphic moonlighting feature of TPPP/p25 has significant innovative impact in anti-Parkinson drug research.

Moonlighting microtubule-associated proteins: regulatory functions by day and pathological functions at night

The sensing, integrating, and coordinating features of the eukaryotic cells are achieved by the complex ultrastructural arrays and multifarious functions of the cytoskeletal network. Cytoskeleton comprises fibrous protein networks of microtubules, actin, and intermediate filaments. These filamentous polymer structures are highly dynamic and undergo constant and rapid reorganization during cellular processes. The microtubular system plays a crucial role in the brain, as it is involved in an enormous number of cellular events including cell differentiation and pathological inclusion formation. These multifarious functions of microtubules can be achieved by their decoration with proteins/enzymes that exert specific effects on the dynamics and organization of the cytoskeleton and mediate distinct functions due to their moonlighting features. This mini-review focuses on two aspects of the microtubule cytoskeleton. On the one hand, we describe the heteroassociation of tubulin/microtubules with metabolic enzymes, which in addition to their catalytic activities stabilize microtubule structures via their cross-linking functions. On the other hand, we focus on the recently identified moonlighting tubulin polymerization promoting protein, TPPP/p25. TPPP/p25 is a microtubule-associated protein and it displays distinct physiological or pathological (aberrant) functions; thus it is a prototype of Neomorphic Moonlighting Proteins. The expression of TPPP/p25 is finely controlled in the human brain; this protein is indispensable for the development of projections of oligodendrocytes that are responsible for the ensheathment of axons. The nonphysiological, higher or lower TPPP/p25 level leads to distinct CNS diseases. Mechanisms contributing to the control of microtubule stability and dynamics by metabolic enzymes and TPPP/p25 will be discussed.

Protein conformational disorder and enzyme catalysis

Though lacking a well-defined three-dimensional structure, intrinsically unstructured proteins are ubiquitous in nature. These molecules play crucial roles in many cellular processes, especially signaling and regulation. Surprisingly, even enzyme catalysis can tolerate substantial disorder. This observation contravenes conventional wisdom but is relevant to an understanding of how protein dynamics modulates enzyme function. This chapter reviews properties and characteristics of disordered proteins, emphasizing examples of enzymes that lack defined structures, and considers implications of structural disorder for catalytic efficiency and evolution.

A new protein superfamily: TPPP-like proteins

The introduction of the term ‘Tubulin Polymerization Promoting Protein (TPPP)-like proteins’ is suggested. They constitute a eukaryotic protein superfamily, characterized by the presence of the p25alpha domain (Pfam05517, IPR008907), and named after the first identified member, TPPP/p25, exhibiting microtubule stabilizing function. TPPP-like proteins can be grouped on the basis of two characteristics: the length of their p25alpha domain, which can be long, short, truncated or partial, and the presence or absence of additional domain(s). TPPPs, in the strict sense, contain no other domains but one long or short p25alpha one (long- and short-type TPPPs, respectively). Proteins possessing truncated p25alpha domain are first described in this paper. They evolved from the long-type TPPPs and can be considered as arthropod-specific paralogs of long-type TPPPs. Phylogenetic analysis shows that the two groups (long-type and truncated TPPPs) split in the common ancestor of arthropods. Incomplete p25alpha domains can be found in multidomain TPPP-like proteins as well. The various subfamilies occur with a characteristic phyletic distribution: e. g., animal genomes/proteomes contain almost without exception long-type TPPPs; the multidomain apicortins occur almost exclusively in apicomplexan parasites. There are no data about the physiological function of these proteins except two human long-type TPPP paralogs which are involved in developmental processes of the brain and the musculoskeletal system, respectively. I predict that the superfamily members containing long or partial p25alpha domain are often intrinsically disordered proteins, while those with short or truncated domain(s) are structurally ordered. Interestingly, members of this superfamily connected or maybe connected to diseases are intrinsically disordered proteins.

Microtubule assembly-derived by dimerization of TPPP/p25. Evaluation of thermodynamic parameters for multiple equilibrium system from ITC data

BACKGROUND

The disordered Tubulin Polymerization Promoting Protein/p25 (TPPP/p25) modulates the dynamics and stability of the microtubule system. In this paper the role of dimerization in its microtubule-related functions is established, and an approach is proposed to evaluate thermodynamic constants for multiple equilibrium systems from ITC measurements.

METHODS

For structural studies size exclusion chromatography, SDS-PAGE, chemical cross-linking, circular dichroism, fluorescence spectroscopy and isothermal titration calorimetry were used; the functional effect was analyzed by tubulin polymerization assay. Numerical simulation of the multiple equilibrium was performed with Mathematica software.

RESULTS

The dimerization of TPPP/p25 is promoted by elevation of the protein concentration and by GTP addition. The dimeric form displaying enhanced tubulin polymerization promoting activity is stabilized by disulfide bond or chemical cross-linking. The GTP binding to the dimeric form (Kd-GTP=200 μM) is tighter with one order of magnitude than to the monomeric one leading to the enrichment of the dimers. A mathematical model elaborated for the multiple equilibrium of the TPPP/p25-GTP system was validated by fitting the GTP-dependent changes of ellipticity and fluorescence signal in the course of TPPP/p25 titrations. The evaluation of the equilibrium constants rendered it possible to determine the thermodynamic parameters of the association of different TPPP/p25 forms with GTP from ITC measurements.

CONCLUSIONS/GENERAL SIGNIFICANCE

The dimerization of TPPP/p25 with favorable physiological functional potency is proposed to play significant role in the fine tuning of TPPP/p25-mediated microtubule assembly; the unfolded monomers might be involved in the formation of pathological inclusions characteristic for Parkinson's disease and other synucleinopathies.