Tubulin Polymerization Promoting Proteins: Functional Diversity With Implications in Neurological Disorders

Abstract

Tubulin Polymerization Promoting Proteins (TPPPs) are highly conserved across species but remain poorly understood. There are three TPPP genes in humans, with only one homologous TPPP gene in invertebrates, such as Drosophila and C. elegans. The human TPPP (TPPP1/p25/p25α) is enriched in the brain and shares sequence similarities with the invertebrate TPPPs. TPPP/p25 associates with microtubules and plays a pivotal role in microtubule dynamics, bundling, and polymerization, thereby stabilizing the microtubular network. This is essential for cytoskeletal organization and proper functioning of neurons and glial cells, including axonal growth, regeneration, migration, trafficking, synapse formation, and myelination of axons. However, studies have also uncovered that besides its cytoplasmic/microtubular localization, TPPP/p25 is present in other subcellular compartments, including the mitochondria and nucleus, underscoring the presence of additional novel functions. At the molecular level, TPPP/p25 is predicted to exist as an intrinsically disordered protein and is implicated in neurological and neurodegenerative disorders, including Parkinson's and related disorders and Multiple Sclerosis. In this article, we provide a comprehensive overview of TPPP/p25, highlighting its evolutionary conservation, cellular and subcellular localization, established and emerging functions in the nervous system, interacting partners, potential clinical relevance to human neurological disorders, and conclude with unresolved questions and future areas of study.

Keywords: CNS; ensheathment; microtubule; mitochondria; α‐synucleinopathies.

FIGURE 1

Milestones in TPPP research. A schematic presentation of the key milestones in TPPP research since its initial discovery. Image created with BioRender.com.

FIGURE 2

Evolutionary conservation of TPPPs across animal phyla. TPPP is conserved across all major animal phyla from Porifera to vertebrates highlighting its essential biological functions during animal evolution.

FIGURE 3

Multiple sequence alignment of TPPP‐family proteins across diverse species. Protein sequences from human (TPPP1), Strongylocentrotus purpuratus, Crassostrea virginica, Drosophila melanogaster, Capitella teleta, Caenorhabditis elegans, Schistosoma japonicum, Nematostella vectensis, and Suberites domuncula were aligned using SnapGene software. The alignment was performed using the integrated multiple sequence alignment tool and visualized within the SnapGene protein viewer. Highlighted residues (in yellow) indicate positions conserved among multiple species but not necessarily matching the consensus sequence. Gaps introduced to optimize the alignment are indicated by dashes (–). The colored bars above the sequences represent conservation scores: Dark red blocks indicate regions of high sequence conservation, whereas lighter blue/yellow/orange blocks represent regions of moderate to low conservation. Regions without color or with dark blue indicate no strong conservation across aligned sequences. This visualization highlights highly conserved regions within the TPPP protein family, particularly emphasizing the central microtubule binding (or p25α) domain, suggesting significant functional importance despite evolutionary divergence. Within the p25α domain, the GXGXGXXGR motif stands out as a highly conserved sequence, indicative of a Rossmann‐like fold. The conservation of this motif across species implies a fundamental role in the protein's function.

FIGURE 4

Protein sequence of human TPPP/p25. Presence of specific motifs in human TPPP corresponding to a phosphorylation site (green, TPPKSP) within this segment is also present in tau protein; LC3B‐interacting region (LIR) motif (blue, WSKL and FSKI), and a Rossmann motif (red, GKGKGKRGR).

FIGURE 5

TPPP localization in the nervous system across species. A schematic illustration of the various cell types and subcellular compartments that TPPP localizes to.

FIGURE 6

Diverse functions performed by TPPP. TPPP exhibits a broad range of functions in the nervous system across species. Under normal physiological conditions, it is involved in microtubule bundling/polymerization and stability, oligodendrocyte differentiation/myelination, neuronal survival, axon growth/regeneration, and synaptic organization and growth. TPPP is also pro‐aggregatory and promotes α‐Synuclein aggregation under pathological conditions.

FIGURE 7

Disorder prediction in human TPPP and α‐Synuclein and their co‐aggregation in Lewy bodies of Parkinson's disease. (a, b) The predicted disorder in the full‐length human TPPP (a) and α‐Synuclein (b) by PONDR (Predictor of Natural Disordered Region; Orosz et al. 2004). Protein sequences were obtained from NCBI database. The accession number of human TPPP is O94811.1 and human α‐Synuclein is P37840.1. (c–c″) Confocal images from a Parkinson's disease patient substantia nigra brain region labeled with human anti‐TPPP (green, c, c″) and anti‐aggregated α‐Synuclein (red, c′, c″) antibodies show co‐aggregation of TPPP and α‐Synuclein in Lewy bodies. Anti‐TPPP (Kovacs et al. 2004) and anti‐aggregated α‐Synuclein (Abcam, ab209538) were used at 1:800 and 1:400 dilutions, respectively. Secondary antibodies conjugated to Alexa‐488 and Alexa‐568 (Invitrogen) were used at 1:400 dilution. Scale bar in c–c″ = 10 μm.