MYL12B

MYL12B is a non-muscle myosin II regulatory light chain that assembles with the non-muscle myosin heavy chains MYH9 and MYH10 and the essential light chain MYL6 to form the non-muscle myosin II complex, where it is required to maintain the stability of the holoenzyme and normal cell morphology and dynamics (PMID:21126233). Its activity is controlled by phosphorylation: smooth muscle myosin light chain kinase (smMLCK) phosphorylates MYL12B in vitro, and in cochlear hair cells this phosphorylation drives apical constriction-like shape changes that are reversed by smMLCK inhibition (PMID:33877471). Beyond conventional myosin II, MYL12B also serves as a functional regulatory light chain for the unconventional motor Myosin 5c, supporting its actin-activated ATPase and motile activity (PMID:38606007). At the pathway level, MYL12B couples to vascular smooth muscle cell identity, as its suppression by miR-23a-3p blocks MRTFA nuclear translocation and promotes phenotypic switching (PMID:41784666).

1. 2011 High

   Established that MYL12B is a bona fide subunit of the non-muscle myosin II complex and that regulatory light chains are structurally required to keep the holoenzyme stable, rather than being passive accessory proteins.

   Evidence Proteomic co-IP/pulldown plus siRNA knockdown with western blot and morphological readouts in NIH 3T3 fibroblasts

   PMID:21126233

   Open questions at the time

   • Does not distinguish the individual contribution of MYL12B from MYL12A or MYL9
   • Mechanism by which RLC loss destabilizes heavy chains not defined
2. 2015 Medium

   Showed that phosphorylation, not total abundance, is the regulated state of MYL12B in a disease context, pointing to phospho-MYL12B as the functionally relevant species.

   Evidence 2D-DIGE proteomics with MS identification and western blot validation in a septic acute kidney injury mouse model

   PMID:26823757

   Open questions at the time

   • Single proteomic study without mechanistic dissection of the kinase or downstream effect
   • Causal role of phospho-MYL12B in kidney injury not tested
3. 2021 High

   Identified smMLCK as a direct kinase for MYL12B and linked its phosphorylation to actomyosin-driven cell shape change, connecting molecular activation to a defined morphological output in hair cells.

   Evidence In vitro kinase assay with purified MYL12B and smMLCK plus ML-7 pharmacological inhibition with immunofluorescence in cochlear hair cells

   PMID:33877471

   Open questions at the time

   • Phospho-site on MYL12B not mapped
   • ML-7 is not fully specific to smMLCK
4. 2021 Medium

   Reinforced MYL12B's structural role by showing that an lncRNA scaffold is needed to hold the NM IIA subunits together and sustain heavy-chain ATPase activity.

   Evidence RNA pulldown/co-IP of MAFG-AS1 with MYL12B/MYH9/MYL6 and ATPase assay after MAFG-AS1 knockdown

   PMID:33813778

   Open questions at the time

   • Effect on MYL12B is indirect through the complex rather than MYL12B-specific
   • Single lab, no reciprocal validation of the MYL12B contact
5. 2024 Medium

   Expanded MYL12B's role beyond conventional myosin II by demonstrating it functions as a regulatory light chain for the unconventional motor Myosin 5c.

   Evidence Recombinant co-expression/purification of human Myo5c-HMM with Myl6 and Myl12b, actin-activated ATPase and in vitro motility assays

   PMID:38606007

   Open questions at the time

   • MYL12B's specific contribution not isolated by mutagenesis
   • Physiological context of Myo5c regulation by MYL12B unknown
6. 2024 Medium

   Placed MYL12B within a vascular signaling axis, showing its miR-23a-3p-mediated suppression controls MRTFA nuclear translocation and smooth muscle cell phenotype.

   Evidence miRNA target validation, engineered EVs loaded with miR-23a-3p, immunofluorescence and in vivo carotid artery model in Ldlr KO rats

   PMID:41784666

   Open questions at the time

   • Mechanistic link from MYL12B to MRTFA localization not resolved
   • Single lab, target relationship not orthogonally confirmed

• How phosphorylation, motor partner selection (myosin II vs Myo5c), and transcriptional coupling to MRTFA are integrated by MYL12B remains unresolved.
• No structural model of MYL12B bound to its distinct heavy-chain partners
• Phospho-site identity and its differential effect across myosin classes unmapped
• Mechanistic chain from MYL12B level to MRTFA nuclear translocation undefined