KY

KY encodes a transglutaminase-like protease expressed specifically in skeletal muscle and heart that is essential for normal muscle growth, function, and maturation of the neuromuscular junction, with its loss producing a postural muscle dystrophy (PMID:11136708). KY localizes to the sarcomeric Z-band, where it assembles with IGFN1 and filamin C (FLNC) into a Z-band-associated complex that provides structural support to the sarcomere (PMID:20206623). Mechanistically, KY exhibits intrinsic protease activity, binds FLNC, and proteolytically degrades filamin C in cells, and its loss in vivo produces irregular FLNC subcellular distribution, establishing KY as a regulator of filamin C localization and turnover (PMID:15385448). Consistent with a sarcomeric maintenance role, KY deficiency triggers constitutive upregulation of titin-based signalling proteins (MLP, MARP2, Xin), pointing to titin structural/signalling instability as an early consequence of KY loss (PMID:16806927), and drives upregulation of BAG3 and other chaperone-assisted selective autophagy factors with impaired turnover, indicating that FLNC clearance via CASA is dysregulated when KY is absent (PMID:29914939). In humans, biallelic loss-of-function mutation in KY causes an early-onset neuromuscular disorder with nemaline rods and thickened Z-discs, accompanied by abnormal FLNC and Xin distribution that mirrors the mouse phenotype (PMID:27485408).

1. 1998 Low

   Before KY had a molecular identity, the ky mouse mutant locus needed to be cloned; mapping localized the gene to a small chromosome 9 interval and pinpointed a muscle-specific candidate transcription unit.

   Evidence YAC/BAC contig construction and STS mapping with expression screening in mouse

   PMID:9878244

   Open questions at the time

   • No functional or mechanistic data on the candidate gene
   • Protein product and activity unknown at this stage
2. 2001 High

   Positional cloning identified KY as a novel muscle- and heart-specific gene and established that its loss impairs hypertrophic muscle growth and neuromuscular junction maturation, defining its physiological importance.

   Evidence Positional cloning, expression analysis, and histopathology of ky/ky null muscle including NMJ analysis

   PMID:11136708

   Open questions at the time

   • Molecular activity of the KY protein not yet determined
   • Direct binding partners and substrates unidentified
3. 2002 Medium

   Identification of the C. elegans homologue ltd-1 with LIM and transglutaminase domains established that the transglutaminase-like domain is an evolutionarily ancient, conserved feature of the protein family.

   Evidence Cloning of the C. elegans ortholog and GFP reporter expression during development

   PMID:12204272

   Open questions at the time

   • Catalytic activity of the conserved domain not demonstrated in this work
   • Functional relationship of hypodermal expression to mammalian muscle role unclear
4. 2004 High

   To define KY's biochemical function, interaction and activity assays showed KY binds filamin C, has protease activity, degrades filamin C in cells, and that FLNC is mislocalized in KY-null muscle, establishing KY as a protease regulating filamin C.

   Evidence In vitro binding/pull-down, in vitro protease assays, transfected-cell degradation assays, and immunofluorescence of ky/ky muscle

   PMID:15385448

   Open questions at the time

   • Endogenous physiological substrates beyond filamin C not enumerated
   • Catalytic residues and cleavage site not mapped
5. 2004 Medium

   Molecular profiling of dystrophic ky/ky soleus revealed a UCP1/UCP2 signature linked spatially to disorganized acetylcholine receptor clusters and disrupted MuSK-dependent signalling, connecting KY loss to NMJ signalling breakdown.

   Evidence Microarray profiling of EDL and soleus with immunofluorescence and protein validation

   PMID:15036332

   Open questions at the time

   • Causal link between KY protease activity and NMJ/UCP changes not established
   • Whether UCP upregulation is primary or secondary unresolved
6. 2006 Medium

   Examining downstream signalling showed titin-based proteins MLP, MARP2, and Xin are constitutively upregulated in KY-deficient muscle, implicating titin structural/signalling instability as an early consequence of KY loss.

   Evidence Protein-level expression analysis in ky/ky fast and slow muscles with comparison to other disease models

   PMID:16806927

   Open questions at the time

   • Direct mechanistic link between KY and titin signalling not shown
   • Single-lab characterization without functional rescue
7. 2010 High

   Localization and interaction studies placed KY at the Z-band in a complex with IGFN1 and FLNC, defining a structural sarcomeric module rather than a purely catalytic role.

   Evidence Endogenous immunodetection in C2C12 myotubes, expression in cardiomyocytes, yeast two-hybrid, biochemistry, and immunofluorescence

   PMID:20206623

   Open questions at the time

   • Stoichiometry and assembly order of the KY–IGFN1–FLNC complex unknown
   • Whether IGFN1 is a substrate or scaffold not resolved
8. 2012 Low

   Expression profiling of CAPN3-knockout muscle found KY downregulated, hinting at a complementary role for KY protease activity in cytoskeleton homeostasis during changes in muscle activity.

   Evidence Microarray of C3KO soleus versus wild-type

   PMID:22820870

   Open questions at the time

   • Expression correlation only, no functional validation of consequence
   • Direct CAPN3–KY mechanistic interaction not tested
9. 2016 Medium

   A homozygous KY truncation in patients with an early-onset neuromuscular disorder showing nemaline rods, thickened Z-discs, and FLNC/Xin mislocalization translated the mouse mechanism to human disease.

   Evidence Whole-exome sequencing and muscle biopsy immunofluorescence

   PMID:27485408

   Open questions at the time

   • Genotype–phenotype range across patients not defined
   • Residual protease function of the truncation not assessed
10. 2018 Medium

    CRISPR loss-of-function in C2C12 cells and zebrafish, plus mouse data, showed KY deficiency upregulates BAG3 and CASA factors and impairs their stress-induced adaptation, linking KY to chaperone-assisted selective autophagy-mediated FLNC turnover.

    Evidence CRISPR/Cas9 knockout in cells and zebrafish, swimming-challenge assay, and protein analysis in ky/ky mice

    PMID:29914939

    Open questions at the time

    • Direct molecular link between KY protease activity and CASA induction unresolved
    • Whether CASA upregulation compensates for or contributes to pathology unclear

• How KY protease activity is regulated and how its cleavage of filamin C is mechanistically coupled to titin signalling, NMJ stability, and CASA induction remains unresolved.
• No structural model of the KY catalytic domain or cleavage mechanism
• Catalytic residues and physiological substrate repertoire incompletely defined
• Direct mechanistic chain from KY loss to titin/CASA/NMJ phenotypes not established