Tubulin Folding and Degradation

Abstract

Synthesis of polypeptides inside a cell is an astonishing, complex and very efficient process. To reach the active state, proteins will have to face many different problems that in part are solved by molecular chaperones. Tubulins belong to one of the largest superfamilies of proteins with six conserved members. Different members have different roles although they share a general fold. Tubulins reach their final structure in an intricate pathway where different molecular chaperones are involved. Prefoldin and cytosolic chaperonin containing TCP1/TriC are required in the first part of the pathway, while tubulin folding cofactors (TBCs) are involved in the formation of the α‐ and β‐tubulin heterodimers. Later, their role in tubulin proteostasis as well as their implications in different neurodevelopmental disorders has been documented.

Key Concepts

  • Tubulins represent one of the largest families of genes and proteins.
  • α‐ and β‐Tubulin heterodimers constitute the structural and functional subunit of microtubules (MTs).
  • Folding of tubulins follows a sophisticated pathway involving several molecular chaperones.
  • Formation of the αβ‐tubulin heterodimer is an extremely regulated process for it is involved in different lateral and longitudinal as well as microtubule associated proteins (MAPs) interactions.
  • TBCs are molecular chaperones specifically required for αβ‐tubulin dimer formation.
  • TBCs are shown to be involved in the release of tubulin monomers from the chaperonin during the folding pathway.
  • The dissociation constant for the tubulin heterodimer has shown to be 10−11 M and TBCs are required for an efficient tubulin dimer dissociation.
  • TBCs are involved in tubulin proteostasis through dimer dissociation and tagging for proteasome recognition.
  • The first 3D structure of a complex formed between α‐tubulin and two chaperones involved in the tubulin turnover cycle has been established.
  • Genetic defects on TBCs are responsible of neurodevelopmental diseases.

Keywords: tubulin folding cofactors (TBCs); tubulin heterodimer; microtubule dynamics; degradation; ubiquitin; proteasome; proteostasis; molecular chaperones

Figure 1. Network of molecular chaperones involved in α‐ and β‐tubulin folding. (a) The tubulin folding pathway starts when the newly synthetised tubulin polypeptide is captured by prefoldin. This cochaperone protects the polypeptide from unwanted interactions, and it is delivered to the cytosolic chaperonin CCT. Upon several rounds of ATP hydrolysis, CCT undergoes a conformation change from a close to an open conformation that allows the tubulin monomer to achieve its final 3D structure within the CCT cavity. (b–e) Atomic structure of the molecular chaperones involved in this pathway; including the archaeal homologue of prefoldin (b, PDB ID: 1FXK), the opened (c, modify from PDB ID: 2XSM) and closed conformation of the chaperonin CCT (d, PDB ID: 4V8R) and the proposed model for the unfolded tubulin (green, modify from PDB ID: 1TUB) delivery to the CCT (salmon, modify from PDB ID: 2XSM) by prefoldin (blue, PDB ID: 1FXK). Scale bar 20 Å. Based on Martín‐Benito et al.,2002 © National Institutes of Health.
Figure 2. Tubulin binding cofactors define a postchaperonin pathway that give rise to the αβ‐tubulin dimer assembly. (a) A tubulin dimer is composed of α‐ and β‐tubulin monomers whose final 3D structure is almost identical (right panel showing α‐helices in blue and β‐sheets in orange). In both monomers, three different protein domains can be defined based on their functionality: an ‐terminal domain (green), an intermediate domain (orange) and a ‐terminal domain (blue) (left panel). (b) Folded α‐ and β‐tubulin monomers released from CCT by different cofactors are finally captured by TBCE or TBCB and TBCA or TBCD, respectively. Binding of TBCC to the supercomplex composed of α‐tubulin:TBCE and β‐tubulin:TBCD stimulates GTP hydrolysis in the β‐tubulin subunit. Consecutively, a functional αβ‐tubulin dimer competent for microtubule assembly is released.
Figure 3. Available atomic structures of the tubulin‐binding cofactors and/or homologues protein domains. (a) TBCA shows an all α‐helical three‐dimensional structure (PDB ID: 1H7C). (b) TBCD is composed of a number of Armadillo and HEAT repeat domains (PDB IDs: 1EJL and 1QBK). (c) TBCC contains a Spectrin ‐terminal domain and a RP2 ‐terminal domain (PDB IDs: 2L3L and 1K4Z). (d) Domain composition of TBCB includes a CAP‐Gly (PDB ID: 1WHG) and a UBL domain (PDB ID: 1V6E). (e) TBCE contains also a CAP‐Gly (2MPZ) and a UBL (4ICU) domain with an intermediate domain that contains leucine‐rich repeats (PDB ID: 3RJ0).
Figure 4. Tubulin‐binding cofactors are involved in αβ‐tubulin dimer dissociation upon microtubule depolymerisation. Tubulin dimers released from the microtubule are captured by TBCs that dissociate them. Depending on the tubulin monomer, one or the other will be retained by TBCD, in the case of β‐tubulin, or by the complex TBCB/TBCE, in the case of α‐tubulin. Both TBCs pathways may lead to the tubulin monomer degradation through the ubiquitin‐proteasome system by an unknown mechanism.
Figure 5. The αEB complex is the central protein complex in the α‐tubulin turnover and degradation pathway. (a) Three‐dimensional structure of TBCE. (b) Three‐dimensional structure of the ternary αEB complex composed of TBCE (blue), TBCB (yellow) and α‐tubulin (pink). (c) Molecular architecture of TBCE. The protein is composed of CAP‐Gly (light blue, PDB ID: 1WHG), LRR (orange, PDB ID: 3RJ0) and UBL (green, PDB ID: 4ICU) domains. (d) Molecular architecture of the αEB complex. This tertiary complex is composed of the three protein domains of TBCE defined in (c); the UBL (magenta, PDB ID: 1V6E) and CAP‐Gly (dark blue, PDB ID: 1WHG) domains of TBCB; and the α‐tubulin monomer (yellow, PDB ID: 1TUB). (e) Molecular mechanism proposed for the αβ‐tubulin dimer dissociation by TBCE and TBCB. The heterodimer dissociation is an energy‐independent process where the LRR domain of TBCE (orange) might disrupt the αβ‐tubulin dimer interface, displace the β‐tubulin monomer (blue) and keep the α‐tubulin monomer protected in a stable αEB complex.
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Serna, Marina, and Zabala, Juan C(Apr 2016) Tubulin Folding and Degradation. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0026333]