MYL6B

MYL6B encodes a myosin alkali essential light chain that arose by gene duplication from MYL6 and is expressed in both non-muscle cells and slow skeletal/cardiac muscle, where it incorporates into myosin II holoenzymes and modulates shortening velocity and isometric force generation (PMID:2304459, PMID:16884681). Its N-terminal lobe dictates isoform-specific subcellular sorting to sarcomeric A-bands in cardiomyocytes, and it binds the first IQ motif of IQGAP1 and IQGAP3 scaffold proteins (PMID:8856505, PMID:18587628, PMID:21299499). Within non-muscle myosin II, MYL6B bridges MDM2 and p53, promoting MDM2-mediated ubiquitination and proteasomal degradation of p53 in an ATPase-dependent manner, thereby suppressing apoptosis in hepatocellular carcinoma cells (PMID:29439719). MYL6B also drives epithelial–mesenchymal transition, proliferation, migration, and invasion in colorectal and lung cancer cells, and its expression is regulated by the circ_0114866/miR-653-5p ceRNA axis (PMID:33817240, PMID:39281569).

1. 1990 Medium

   Establishing that MYL6B is a distinct alkali light chain isoform with non-muscle expression resolved the question of whether multiple alkali MLC genes encode functionally divergent proteins beyond the known skeletal/cardiac forms.

   Evidence cDNA cloning, Northern/Southern blotting, and cross-species expression profiling

   PMID:2304459

   Open questions at the time

   • No functional assay distinguishing MYL6B from MYL6 at the protein level
   • Tissue-specific regulation of MYL6B transcription not defined
2. 1996 High

   Demonstrating that MYL6B sorts to sarcomeric A-bands with isoform-specific efficiency, and that the N-terminal lobe controls this sorting, established the first structure–function relationship for MYL6B within the myosin holoenzyme.

   Evidence Chimeric cDNA expression with double epitope-tagging competition assay and confocal microscopy in cultured rat cardiomyocytes

   PMID:8856505

   Open questions at the time

   • Mechanism by which the N-terminal lobe achieves sorting selectivity not resolved at atomic level
   • Whether sorting differences translate into distinct contractile outputs in vivo unknown
3. 2006 Medium

   Correlating MYL6B abundance with decreased maximal shortening velocity and increased isometric force in single muscle fibers provided the first direct evidence that this isoform tunes mechanical performance of myosin II.

   Evidence Single-fiber mechanics (Vmax, Po/CSA) with protein isoform quantification in pig diaphragm slow fibers

   PMID:16884681

   Open questions at the time

   • Correlative design; no gain/loss-of-function manipulation of MYL6B in intact fibers
   • Whether these mechanical effects hold in non-muscle myosin II contexts untested
4. 2008 Medium

   Identifying MYL6B as a specific binding partner of the first IQ motif of IQGAP1 expanded its interactome beyond myosin heavy chains to cytoskeletal scaffolding proteins.

   Evidence Native gel electrophoresis with synthetic IQ-motif peptides of IQGAP1

   PMID:18587628

   Open questions at the time

   • Interaction shown only with isolated peptides; not confirmed by co-IP or in intact cells
   • Functional consequence of MYL6B–IQGAP1 binding unknown
5. 2011 Medium

   Extending the MYL6B–IQGAP interaction to IQGAP3's first IQ motif established IQ-motif selectivity across the IQGAP family, though the physiological role remained untested in mammalian cells.

   Evidence Native gel electrophoresis with synthetic IQ-motif peptides from IQGAP2 and IQGAP3; review synthesis with yeast analogy

   PMID:21299499 PMID:21428964

   Open questions at the time

   • No mammalian cell-based validation of functional relevance (e.g., cytokinesis)
   • Affinity and stoichiometry of IQGAP–MYL6B complexes not quantified
6. 2018 High

   Discovering that MYL6B, as part of non-muscle myosin II, bridges MDM2 and p53 to promote p53 ubiquitination and degradation revealed an unexpected ATPase-dependent role for a structural myosin subunit in oncogenic signaling.

   Evidence Co-IP, mass spectrometry, ubiquitination assay, blebbistatin-mediated ATPase inhibition, MYL6B knockout in HCC cell lines with apoptosis and clonogenic readouts

   PMID:29439719

   Open questions at the time

   • Structural basis for how MYL6B simultaneously contacts MDM2 and p53 unknown
   • Whether this mechanism operates in non-cancer physiological contexts untested
   • Relative contribution of MYL6B versus MYL6 in NMII-dependent p53 regulation not addressed
7. 2020 Medium

   Linking MYL6B knockdown to reversal of EMT markers and suppression of proliferation/migration in rectal adenocarcinoma cells positioned MYL6B as a functional driver of epithelial–mesenchymal transition in cancer.

   Evidence siRNA knockdown with CCK-8, Transwell, flow cytometry, and Western blot for E-cadherin, N-cadherin, Vimentin in rectal adenocarcinoma cells

   PMID:33817240

   Open questions at the time

   • Downstream signaling pathway connecting MYL6B to EMT transcription factor activation not identified
   • Single cancer type tested; generalizability uncertain
8. 2024 Medium

   Identifying circ_0114866/miR-653-5p as an upstream ceRNA axis that de-represses MYL6B provided the first regulatory circuit controlling MYL6B expression in cancer and confirmed its oncogenic role in vivo.

   Evidence Dual-luciferase reporter assay, qPCR, Western blot, functional assays, and xenograft tumor model in NSCLC

   PMID:39281569

   Open questions at the time

   • Whether miR-653-5p regulates MYL6B in normal tissue homeostasis unknown
   • Transcriptional regulation of MYL6B independent of miRNA sponging not explored

• The structural basis for MYL6B's specificity within NMII holoenzymes, the mechanism linking its ATPase-dependent function to MDM2–p53 complex formation, and its physiological roles outside cancer contexts remain unresolved.
• No crystal or cryo-EM structure of MYL6B in complex with myosin heavy chain or MDM2/p53
• In vivo genetic models (knockout/knock-in mice) for MYL6B are lacking
• Relative functional redundancy with MYL6 not systematically addressed