The cytoskeleton of eukaryotic cells is a dynamic network based on the cross-talk between three types of filaments: microtubules, actin, and intermediate filaments. This network interacts with a high number of associated proteins that regulate and modulate filaments assembly and dynamics. Altogether, they have multiple roles in cells, such as cell size and shape regulation, cell-cell communication, cell migration, cell division, intracellular localization and coordination of organelles functions, and integration of extracellular signals. Moreover, the cytoskeleton also allows the constant flux of molecules, promoting the interaction between proteins, substrates, and cofactors (Fletcher and Mullins 2010; Fischer and Fowler 2015).
The Microtubule Cytoskeleton Dynamics
As one of the cytoskeleton components, microtubules play critical roles in a range of cellular processes. They function as “highways” to the intracellular transport, allowing the movement of molecules, organelles, proteins, and other cellular components. Microtubules also take part in the segregation of chromosomes during cell division and have an important role in cell shape maintenance and polarity establishment. Furthermore, microtubules are the main structural components of motile and primary cilia. Motile cilia are involved in the generation of cell movement, and both types of cilia have also sensory functions (Fletcher and Mullins 2010; Kollman et al. 2011).
Microtubules are hollow, cylindrical polymers composed of α/β-tubulin heterodimers that polymerize end-to-end into protofilaments. In general, 13 protofilaments associate laterally to form the wall of the microtubule, generally with 25 nm of diameter (Kollman et al. 2011).
Both α- and β-tubulins are globular proteins with a molecular mass of approximately 50 kDa. Their quaternary structure is similar and they share three functional domains: the N-terminal domain, that binds to the nucleotide guanosine triphosphate (GTP), an intermediate domain, involved in the longitudinal and lateral interactions necessary for microtubule assembly, and the C-terminal domain, responsible for the interaction with several accessory proteins, like MAPs (microtubule associated proteins). The C-terminal domain is also a preferencial region for tubulin post-translational modifications (Kollman et al. 2011; Serna and Zabala 2016). When α- and β-tubulins associate to form the heterodimer, the GTP-binding site of the α-tubulin becomes buried at the interface between tubulins, while the one of β-tubulin remains accessible. Consequently, only β-tubulin can function as a GTPase, catalyzing the hydrolysis of GTP to GDP and an inorganic phosphate group (Pi). The α-tubulin site is designated by nonexchangeable GTP site (N-site), whereas that of β-tubulin is known as exchangeable GTP site (E-site) (Downing and Nogales 1998; Nogales 2001).
Microtubules are intrinsically polarized structures since α/β-tubulin heterodimers are always bound in the same orientation. Consequently, microtubules have two different ends: one that exposes β-tubulin, the (+) end, and the other one that exposes α-tubulin, the (-) end. When both ends are free the (+) end is more dynamic than the (-) end. Both in vivo and in vitro, microtubules are very dynamic since they can stochastically switch between growing and shortening phases. This behavior, known as dynamic instability, is dependent on the binding of GTP to β-tubulin. After polymerizing into the microtubule, this GTP is hydrolyzed to GDP+Pi (Nogales 2001; Kollman et al. 2011).
Briefly, the presence of GTP at the E-site of the β-tubulin subunit is required for polymerization; however, following this the GTP is hydrolyzed. When the rate of polymerization is higher than the velocity of the hydrolysis of GTP, the microtubule grows and forms a GTP-cap at the (+) end of the microtubule. When the hydrolysis of GTP occurs and, consequently, the cap is lost, the microtubule becomes unstable and the protofilaments peel outward, causing the microtubule depolymerization (Serna and Zabala 2016).
In animal cells, the (-) end of the microtubules is anchored at the centrosome, the MTOC (major microtubule organizing center), which controls the nucleation and the polymerization of the (+) end (Kollman et al. 2011). The centrosome is formed by two orthogonally displayed centrioles, surrounded by the pericentriolar matrix (PCM). PCM is a highly ordered distribution of individual components that is critical for centrosome functions (Luders 2012).
The assembly of tubulin heterodimers competent to polymerize in microtubules is a complex process known as tubulin folding pathway that involves several molecular chaperones and a diverse group of tubulin binding proteins.
The Tubulin Folding Pathway
Although this pathway has been extensively studied, probably some important mechanistic details are still missing. For example, at least TBCB and probably other TBCs directly bind to the chaperonin CCT, suggesting that the tubulin subunits will be released from the CCT in association with the specific TBC (Carranza et al. 2013).
On the other hand, TBCs are also involved in tubulin heterodimers degradation and recycling, by a process not yet completely understood. TBCB and TBCE interact with α-tubulin and TBCD with β-tubulin, promoting the recycling and/or degradation of α and β-tubulin subunits. This pathway is also regulated by Arl2, which interacts with TBCD preventing the dissociation of native tubulin heterodimers (Gonçalves et al. 2010b) (see Fig. 1). Synthesis and degradation/recycling of tubulin heterodimers is probably very important for the quality control of tubulin pools and dynamics of microtubule networks.
TBCC Functional Domains
TBCC was purified and identified for the first time from extracts of bovine testis as a key protein for the release of the heterodimer of tubulin from the supercomplex of α-tubulin/TBCE/β-tubulin/TBCD/TBCC in vitro (Tian et al. 1996). Later it was shown that TBCC is a GAP for β-tubulin enhancing its GTPase activity, which allows the release of native tubulin heterodimers from the supercomplex (see above). Furthermore, although not being crucial, cofactor E improves this process (Tian et al. 1999).
Two functional domains were described in the C-terminal region of TBCC, namely the TBCC (residues 211-248) and the CARP (248-321) domains. The TBCC domain is associated with tubulin folding pathway and the CARP domain was also described in CAP (cyclase-associated proteins) proteins, which are involved in actin polymerization. These domains are shared by two other identified proteins the TBCCD1 and the RP2. Mutations in RP2 are implicated in the Retinitis Pigmentosa pathology, characterized by a progressive photoreceptor cell degeneration (Schwahn et al. 1998), and TBCCD1 is involved in the centrosome-nucleus connection (Gonçalves et al. 2010a) (see TBCCD1 page).
TBCC and RP2 share 53% of similarity and 29% of identity. Similarly to TBCC, RP2 is also a GAP, and this activity of both proteins is related to the TBCC domain, specifically with a conserved arginine residue, in the positions 262 in TBCC and 118 in RP2, that seems to be essential for their activity as GAPs. In both proteins, the mutation of this residue (RP2-R118H and TBCC-R262A) completely abolish their GAP activity. However, these mutations did not change the proteins’ global conformations, supporting the idea that this residue is part of the active center and constitutes an arginine finger, essential for TBCC and RP2 functions as GAPs. Interestingly, RP2 is able to replace TBCC, since studies in Saccharomyces cerevisae showed that RP2 partially reverts the susceptibility to benomyl (a microtubule depolymerizing agent) caused by the knockdown of cin2 (the budding yeast TBCC orthologue) (Bartolini et al. 2002). However, they also showed that, contrary to TBCC, RP2 is not capable of catalyzing tubulin polymerization suggesting that TBCC has an additional role to its GAP activity. On the other hand, TBCCD1 does not share with TBCC and RP2 the conserved residue of arginine, which is in agreement with the fact that TBCCD1 is not able to rescue the phenotype of cin2/TBCC deletion in yeast (Gonçalves et al. 2010a). This result, once again, supports the importance of the Arg residue for TBCC function as a GAP.
TBCC Cellular Localization and Functions
In humans, although TBCC is distributed throughout the retina, a higher expression is detected in the rod and cone photoreceptors. Moreover, TBCC preferentially localizes to the photoreceptor connecting cilium (Grayson et al. 2002).
Although TBCC localization is compatible with its role in tubulin maturation pathway, which is well established, there are some experimental evidences suggesting that this protein may have other functions related to microtubules that still remain elusive. Interestingly, TBCC knockout is lethal in fission yeast, plants, and mammals but not in budding yeast. In Schizosaccharomyces pombe Tbc1 (TBCC orthologue) point mutations cause a severe defect in yeast shape: instead of having a wild-type rod shape, mutants are bent or branched. Also, mutants have defects in the microtubule network, such as broken or shorter microtubules. In these mutant cells tubulin accumulates in the cytoplasm and is not able to polymerize into microtubules (Mori and Toda 2013). Furthermore, in the plant Arabidopsis thaliana, the embryos of Porcino (por; TBCC orthologue) mutants are formed by only a few highly enlarged cells (Steinborn et al. 2002). Also, epidermal cells are enlarged and possess giant nucleus or multiple nuclei suggesting that this protein is required for cell division (Kirik et al. 2002). Surprisingly, there were no detectable changes in microtubules density but its organization was affected.
The impact of TBCC levels in cell cycle progression is complex. In human MCF-7 cell lines (derived from breast cancer cell line) TBCC overexpression affects the distribution of cells throughout cell cycle. A decrease on the percentage of cells in S-phase and an increase on the percentage of cells in G2-M phase was observed, probably due to slowly progression into mitosis (Hage-Sleiman et al. 2010). Interestingly, these authors also observed that TBCC overexpression in implanted xenografts causes a significant decrease on tumor growth and enhanced sensitivity to antimitotic agents (Hage-Sleiman et al. 2010). The overexpression of TBCC in HeLa cells leads to an accumulation of this protein at spindle pole bodies and occasional multipolar spindles (Garcia-Mayoral et al. 2011). These authors suggested that these observations support the G2-M phase blockage observed in TBCC overexpressing MCF-7 cells (Hage-Sleiman et al. 2010; Garcia-Mayoral et al. 2011). On the other hand, TBCC depletion in HeLa cells causes an arrest of cells at metaphase and aberrant mitotic figures mostly due to multipolar spindles (Garcia-Mayoral et al. 2011). Therefore, both TBCC depletion and overexpression causes cell division defects. However, the alpha-tubulin levels did not seem to be affected neither in TBCC overexpressing nor in knockdown cells. Concerning the β-tubulin levels the available data are not clear (Garcia-Mayoral et al. 2011; Hage-Sleiman et al. 2011), although they seem to increase in TBCC overexpression background (Hage-Sleiman et al. 2010). Furthermore, TBCC overexpression causes an increase of Arl2, TBCD, and also acetylated α-tubulin (Hage-Sleiman et al. 2010). By contrast, TBCC knockdown causes a decrease on the levels of the mentioned proteins apart from TBCD, which levels did not seem to be affected (Hage-Sleiman et al. 2011). These authors suggested that TBCC affects microtubule dynamics by measuring different dynamic microtubule parameters as well as by evaluating the availability of polymerizable tubulin pools (Hage-sleiman et al. 2010; Hage-sleiman et al. 2011).
More recently, in fission yeast, the GAP activity of TBCC was extended to Alp41/Arl2, a highly conserved small GTPase (Mori and Toda 2013). Tbc1/TBCC regulates the balance between Alp41/Arl2 -GTP and -GDP bounded states by inactivating Alp41/Arl2 (Alp41/Arl2 -GDP form). This balance is crucial for the maintenance of cell functions since deregulation in favor of either the two forms (-GDP or -GTP) is toxic. Mutants for Alp41/Arl2, unable to exchange the nucleotide, accumulate either in active or inactive form and show the same phenotype of Tbc1/TBCC mutants. In these mutants, microtubules are shorter or severely lost, and tubulin accumulates in dots around the nuclear envelope. Interestingly, Alp1D (TBCD homologous) is able to interact with Alp41/Arl2 bound to GDP. This avoids microtubule depolymerization by Alp1D/TBCD that, in overexpression, co-localizes with microtubules. Therefore, in yeast, Tbc1/TBCC controls microtubule dynamics by regulating the amount of active/inactive Alp41/Arl2, which in turns regulates Alp1D/TBCD (Fig. 1) (Mori and Toda 2013). Thus, TBCC is emerging in yeast, plants, and humans as a protein not only required for tubulin heterodimer maturation but also as a key factor in the regulation of microtubule dynamics.
TBCC is a GAP protein for β-tubulin and Arl2 and an essential protein for the correct α/β-tubulin heterodimer formation. Altered levels of TBCC affect cell cycle progression, cell proliferation, and microtubules dynamics.
TBCC role in the tubulin folding pathway is well established. However, new data suggest that this protein is also an important regulator of tubulin dynamics by mechanisms not yet completely understood. Therefore, a better characterization of this protein and the identification of its partners are needed in order to elucidate through which pathways TBCC regulates the microtubule cytoskeleton assembly and dynamics.
Modifications on the microtubule networks have been correlated with tumor cell aggressiveness phenotype. Since TBCC affects tumor growth and is involved in microtubule dynamics, determining its cellular functions and how this protein is regulated can contribute to improve our knowledge of cancer development. Moreover, it is well established that cancer cells become resistant to many anticancer drugs targeting microtubules by altering microtubule dynamics through the modulation of differential expression of tubulin isotypes and probably adjusting the control of other factors. In this context, the study of TBCC emerges as a promising field to understand and overcome the development of resistance against anticancer drugs. The impact of studying TBCC will go beyond cancer biology since alterations on microtubule structures and dynamics are at the molecular basis of numerous human pathologies, like neurodegenerative diseases.
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