TBC1D22A

TBC1D22A is a putative Rab33 GTPase-activating protein (RabGAP), annotated by virtue of its TBC domain, that functions at the Golgi through interaction with the scaffolding protein ACBD3 (GCP60) (PMID:23572552). It binds ACBD3 at the same site used by the lipid kinase PI4KB, and these interactions are mutually exclusive, positioning TBC1D22A as a competitive regulator of PI4KB recruitment to the Golgi (PMID:23572552). This competition is differentially modulated by viral 3A replication proteins, with enteroviral and kobuviral 3A proteins distinctly affecting TBC1D22A displacement from ACBD3, implicating it in the membrane remodeling co-opted during picornaviral replication (PMID:23572552). Unlike its paralog TBC1D22B, which inhibits ER-to-Golgi transport in a GAP-dependent manner, TBC1D22A does not share this trafficking-inhibitory activity, indicating functional divergence between the paralogs (PMID:40878439). Beyond these ACBD3-centered and paralog-comparison findings, the catalytic GAP activity of TBC1D22A on Rab substrates and its broader cellular roles have not been directly characterized in the available corpus.

1. 2013 Medium

   Establishing what TBC1D22A physically associates with answered whether it operates at the Golgi: it was identified as an ACBD3-interacting factor that competes with PI4KB for the same binding site, defining a competitive recruitment mechanism.

   Evidence Affinity purification-mass spectrometry, fine-scale domain mapping, and competition pulldown in mammalian cells

   PMID:23572552

   Open questions at the time

   • Functional consequence of PI4KB displacement on Golgi PI4P levels not demonstrated
   • Direct binding shown by AP-MS/pulldown but not by reconstitution with purified components
   • Rab GAP substrate not identified
2. 2013 Medium

   Characterizing how the ACBD3 interaction is regulated answered whether viral factors hijack it: enteroviral versus kobuviral 3A proteins differentially displace TBC1D22A, linking the competition to virus-specific Golgi remodeling.

   Evidence Mammalian two-hybrid assay and affinity purification with viral 3A proteins

   PMID:23572552

   Open questions at the time

   • RabGAP activity remains an annotation based on the TBC domain, not enzymatically demonstrated
   • Physiological role of TBC1D22A in uninfected cells not established
   • Whether displacement alters viral replication outcomes not tested directly
3. 2025 Medium

   Comparing the paralogs answered whether TBC1D22A regulates secretory trafficking like TBC1D22B: it does not inhibit ER-to-Golgi transport, revealing functional divergence despite shared domain architecture.

   Evidence RUSH trafficking assay with overexpression of TBC1D22A versus TBC1D22B

   PMID:40878439

   Open questions at the time

   • Negative result for TBC1D22A does not define its actual trafficking substrate or pathway
   • Whether endogenous TBC1D22A has any GAP-dependent transport role untested
   • Single-lab overexpression assay; loss-of-function consequences unknown

• The direct catalytic substrate and cellular function of TBC1D22A remain undefined: whether it acts as a bona fide Rab33 GAP and what process its ACBD3 competition with PI4KB ultimately controls is unresolved.
• No enzymatic demonstration of GAP activity on any Rab
• No loss-of-function phenotype reported
• No structural model of the TBC1D22A-ACBD3 interface