HYCC2

HYCC2 (FAM126B) is a scaffold/adaptor that supports plasma-membrane PI4P synthesis by facilitating recruitment of the lipid kinase PI4KIIIα, functioning redundantly with its paralog FAM126A (PMID:38638566). Reciprocal genetic perturbation and inducible degradation in colorectal cancer cells established that dependence on FAM126B emerges specifically when FAM126A expression is lost, defining a redundant module for PI4KIIIα membrane recruitment and PI4P production (PMID:38638566). This redundancy is conserved in vivo: in Drosophila, human FAM126B overexpression fully rescues the glial enrichment defects, disrupted axonal sheaths, and impaired visual function caused by knockdown of Hyccin/FAM126A in glia (PMID:34894559). Beyond this redundant scaffolding role in PI4KIIIα-dependent PI4P synthesis, no further mechanistic detail has been characterized in the available corpus.

1. 2021 Medium

   Whether FAM126B can functionally substitute for FAM126A in supporting the PI4KIIIα complex was unknown; in vivo rescue showed it can compensate for FAM126A loss at the organismal level.

   Evidence Drosophila glial-specific Hyccin/FAM126A knockdown rescued by human FAM126B overexpression, scored for glial enrichment, axonal sheath integrity, and visual function

   PMID:34894559

   Open questions at the time

   • Demonstrated only in a Drosophila glial model, not mammalian tissue
   • Does not pinpoint the biochemical step (recruitment vs. activation) rescued
   • No direct measurement of PI4P or PI4KIIIα localization in the rescue setting
2. 2024 High

   The molecular basis and conditional nature of FAM126B function were undefined; perturbation and degradation showed it recruits PI4KIIIα to the plasma membrane for PI4P synthesis redundantly with FAM126A, with dependency arising specifically upon FAM126A loss.

   Evidence Genetic knockdown/knockout and inducible protein degradation in colorectal cancer cell lines with PI4P synthesis assays and DepMap mining

   PMID:38638566

   Open questions at the time

   • Single-lab study; structural basis of PI4KIIIα recruitment not resolved
   • Direct physical interaction interfaces between FAM126B and PI4KIIIα not mapped
   • Therapeutic exploitation of the synthetic dependency in FAM126A-silenced tumors untested in vivo

• How FAM126B physically engages the PI4KIIIα complex and what distinguishes its scaffolding from FAM126A remains unresolved.
• No structural model of the FAM126B-PI4KIIIα complex
• Mechanism of membrane recruitment not defined at molecular resolution
• Non-redundant or paralog-specific roles, if any, uncharacterized