MYOZ1

MYOZ1 (FATZ) is a multivalent Z-disc scaffold protein that organizes the sarcomeric protein network and regulates calcium signaling during muscle differentiation and structural maintenance (PMID:10984498, PMID:36574402). It binds alpha-actinin-2 through its C-terminal region and integrates additional Z-disc components including gamma-filamin/ABP-L, telethonin, myotilin, and the filamin-C, -A, and -B isoforms, the latter linking the Z-disc to the sarcolemma via filamins and beta1 integrins (PMID:10984498, PMID:16076904). A conserved C-terminal class III PDZ-binding motif (E[ST][DE][DE]L) mediates interaction with the PDZ domains of Enigma-family proteins ZASP/Cypher, ALP, CLP-36, and RIL, and PKA phosphorylation of this C-terminus modulates these contacts (PMID:19047374). Structurally, MYOZ1 forms a tight fuzzy complex with alpha-actinin-2 as a conformational ensemble with a polar architecture that stabilizes alpha-actinin-2 orientation, and it can phase-separate with alpha-actinin-2 to form biomolecular condensates, providing a membraneless interaction hub during myofibrillogenesis (PMID:34049882). MYOZ1 abundance is controlled by the circadian E3 ligase FBXL21, which ubiquitinates it for proteasomal degradation in a manner accelerated by GSK-3beta; through this regulated turnover MYOZ1 inhibits calcineurin/NFAT signaling, and its accumulation upon FBXL21 loss impairs NFAT nuclear localization, NFAT target gene expression, myoblast differentiation, and sarcomere integrity (PMID:36574402).

1. 2000 Medium

   Establishing MYOZ1 as a Z-disc protein, the founding question was what it binds; demonstrating direct interactions with alpha-actinin-2, gamma-filamin, and telethonin defined it as a multivalent scaffold within the Z-disc network.

   Evidence Yeast two-hybrid and GST overlay assays mapping the alpha-actinin-2 SR3-SR4 / FATZ C-terminus interaction

   PMID:10984498

   Open questions at the time

   • Binding shown in vitro and by Y2H without endogenous reconstitution
   • Functional consequence of each interaction in muscle not addressed
   • Stoichiometry and structural basis of binding unresolved
2. 2005 Medium

   Extending the interactome, the question was how MYOZ1 connects the Z-disc to the cell periphery; binding to myotilin and to filamin-A/-B/-C established a molecular link from Z-disc proteins to the sarcolemma through filamins and beta1 integrins.

   Evidence Transfection, biochemical pulldown, and yeast two-hybrid mapping of myotilin and filamin binding regions

   PMID:16076904

   Open questions at the time

   • Whether N- or C-terminal filamin binding occurs simultaneously not resolved
   • Physiological role of the sarcolemmal link not tested in vivo
3. 2008 Medium

   Asking how MYOZ1 interactions are encoded and regulated, identification of a conserved C-terminal class III PDZ-binding motif binding Enigma-family PDZ proteins, plus PKA phosphorylation of this motif, revealed a phospho-regulated docking site shared across the myotilin family.

   Evidence In vitro binding, in vivo co-immunoprecipitation, and phosphorylation assays

   PMID:19047374

   Open questions at the time

   • Functional effect of PKA phosphorylation on Z-disc assembly in vivo not established
   • Kinase signaling upstream of this motif not defined for MYOZ1 specifically
4. 2021 High

   Moving from binary interactions to mechanism, the question was how MYOZ1 physically organizes the Z-disc; structural resolution of a tight fuzzy complex with alpha-actinin-2 and demonstration of phase separation established that MYOZ1 builds a polar, condensate-forming interaction hub.

   Evidence Integrative structural biology (SAXS, NMR, cryo-EM) with cellular validation and phase-separation assays

   PMID:34049882

   Open questions at the time

   • In vivo relevance of condensate formation during myofibrillogenesis not directly demonstrated
   • How other partners are recruited into the condensate not resolved
5. 2022 High

   Addressing how MYOZ1 levels are controlled and what they signal, FBXL21-mediated ubiquitination with GSK-3beta as a co-regulator linked MYOZ1 turnover to calcineurin/NFAT signaling, myoblast differentiation, and sarcomere integrity.

   Evidence Yeast two-hybrid, Co-IP, ubiquitination assay, GSK-3beta modulation, C2C12 KO lines, NFAT imaging, and Psttm mouse model

   PMID:36574402

   Open questions at the time

   • Phosphodegron site on MYOZ1 targeted by GSK-3beta not mapped
   • Mechanism by which MYOZ1 inhibits calcineurin/NFAT not defined at the molecular level

• How the scaffolding, phase-separation, and degradation functions of MYOZ1 are integrated to control sarcomere assembly versus calcium signaling in intact muscle over time remains open.
• No direct test linking condensate formation to NFAT regulation
• Tissue- and fiber-type-specific roles of MYOZ1 turnover not resolved