TBCC

TBCC (tubulin binding cofactor C) is a regulator of tubulin dimer content and microtubule dynamics that constrains microtubule-dependent proliferation programs (PMID:20384997). Overexpression of TBCC shifts the tubulin pool toward the non-polymerizable fraction, depleting polymerizable tubulin dimers, lowering microtubule mass, and reducing microtubule dynamicity in breast cancer cells, with accompanying suppression of cell cycle progression and tumor growth (PMID:20384997). In ovarian cancer cells, TBCC suppresses cell cycle progression and autophagy by lowering CDK4 and LC3BII while raising α/β-tubulin and p62; this activity is held in check by miR-1251-5p, which targets TBCC to reverse these effects, defining a miR-1251-5p/TBCC/α/β-tubulin regulatory axis (PMID:31278033). Beyond these tumor-cell studies and a centriole-linkage role described for the TBCC family member Tbccd1 in Chlamydomonas (PMID:19631545), no further mechanistic detail for canonical TBCC has been characterized in the available corpus.

1. 2009 Medium

   Whether TBCC-family proteins act beyond tubulin folding was unknown; analysis of the TBCC-like protein Tbccd1 established a structural role in linking mother and daughter centrioles and orienting the mitotic spindle.

   Evidence Loss-of-function asq2 mutant analysis with live-cell imaging of centrioles and spindles in Chlamydomonas

   PMID:19631545

   Open questions at the time

   • Concerns a TBCC family member (Tbccd1), not canonical TBCC
   • Molecular mechanism of centriole linkage not defined
   • No demonstration that human TBCC shares this centriole function
2. 2010 Medium

   Whether TBCC influences the cellular balance of tubulin and microtubule behavior was unresolved; overexpression studies showed it increases non-polymerizable tubulin, depletes polymerizable dimers, and dampens microtubule dynamics, establishing TBCC as a regulator of tubulin dimer content.

   Evidence Lysate fractionation, time-lapse microtubule dynamics imaging, flow cytometry cell cycle analysis, and xenograft growth assays in breast cancer cells

   PMID:20384997

   Open questions at the time

   • Relies on overexpression rather than loss-of-function
   • Single-lab study without independent replication
   • Biochemical mechanism of partitioning tubulin into a non-polymerizable fraction not defined
3. 2019 Medium

   How TBCC connects to proliferation and autophagy control, and how it is itself regulated, was unknown; TBCC was placed in a miR-1251-5p/TBCC/α/β-tubulin axis modulating CDK4, LC3BII, and p62 to suppress cell cycle and autophagy.

   Evidence TBCC and miR-1251-5p overexpression/inhibition with western blotting, cell cycle analysis, and xenograft growth in ovarian cancer cells

   PMID:31278033

   Open questions at the time

   • Direct miR-1251-5p binding to TBCC transcript not biochemically mapped
   • Mechanistic link between TBCC and CDK4/LC3BII/p62 changes unresolved
   • Findings confined to overexpression in cancer-cell models

• The biochemical mechanism by which TBCC partitions tubulin into non-polymerizable versus polymerizable pools, and whether human TBCC contributes to centriole or spindle functions, remains undefined.
• No structural or enzymatic mechanism for tubulin dimer regulation
• No loss-of-function characterization of endogenous TBCC
• Centriole role established only for the family member Tbccd1, not TBCC itself