mlcD (Q7Z2B8) — Myosin-II / Cytokinesis Prediction Assessment

Gene: mlcD (UniProt Q7Z2B8, MLCD_DICDI), Dictyostelium discoideum (NCBITaxon:44689) Focus: computational_prediction — hypothesis prediction-myosin-cytokinesis Terms tested: GO:0000281 mitotic cytokinesis; GO:0030837 negative regulation of actin filament polymerization; GO:0043520 regulation of myosin II filament assembly; GO:0071976 protein localization to cell division site. Reference context: doi:10.64898/2026.03.19.712954

Executive Judgment

Verdict: REFUTED (over-annotation / paralog–class misassignment).

BioReason-Pro-SFT predicts myosin-II and cytokinesis regulatory roles for mlcD. Primary biochemistry and the curated UniProt/dictyBase record independently establish that mlcD is the dedicated essential light chain of MyoD, a single-headed class I (unconventional) myosin — not a component of the conventional myosin-II (mhcA) contractile machinery. All four predicted terms belong to the myosin-II system (heavy chain mhcA plus its regulatory light chain and essential light chain), whose bipolar filaments drive cytokinesis. Class I myosins are monomeric, membrane-associated motors that do not self-assemble into filaments, so mlcD cannot mediate or regulate myosin-II filament assembly or contractile-ring–based cytokinesis.

The most important caveat: I could not access the specific reference doi:10.64898/2026.03.19.712954 programmatically, and I did not run a MyoD-null phenotype search successfully (PubMed queries for class-I phenotypes returned no hits in this environment). The refutation nonetheless rests on direct, unambiguous primary evidence (protein sequencing + complex purification) and on the curated functional record.

Evidence Matrix

GO Curation Implications (leads — require curator verification)

• Do NOT add GO:0000281 (mitotic cytokinesis), GO:0030837 (neg. reg. actin filament polymerization), GO:0043520 (regulation of myosin II filament assembly), or GO:0071976 (protein localization to cell division site) to mlcD. These are conventional-myosin-II / contractile-ring functions and are class-inappropriate for a myosin-I light chain.
• GO:0043520 specifically: refuted. The correct effectors of Dictyostelium myosin-II filament assembly are the mhcA heavy chain (regulated by myosin heavy-chain kinases) and its regulatory / essential light chains — not MlcD. If any myosin light chain warrants GO:0043520, it is the myosin-II RLC/ELC paralog, not mlcD.
• Retain / support instead (MF/CC): MF GO:0032036 myosin heavy chain binding (IPI to myoD) and the calmodulin-like light-chain role; CC GO:0016459 myosin complex. Prefer these informative terms over generic "protein binding."
• Class-appropriate BP/CC leads (verify against MyoD phenotype literature): class-I myosin processes at the plasma membrane — macropinocytosis/endocytosis and cortical actin dynamics (consistent with curated macropinocytic cup cytoskeleton and actin wave localizations).

Mechanistic Scope

Direct molecular function of the gene product: MlcD is a calmodulin-like EF-hand essential light chain that binds the IQ motifs in the neck of the class I myosin heavy chain MyoD, stabilizing its lever arm. Its Ca²⁺ affinity is low, so it is not a Ca²⁺ sensor; De La Roche et al. propose it confers Ca²⁺-insensitive regulatory properties distinguishing MyoD from calmodulin-bearing myosin-I. The predicted terms describe downstream cellular processes of a different motor system (myosin-II contractile ring); they are neither the immediate activity of MlcD nor a documented phenotype of it.

Conflicts and Alternatives

• Paralog/class confusion (most likely source of the prediction): "mlc*" myosin light-chain genes form a paralog pool spanning classes. The myosin-II regulatory (mlcR) and essential (mlcE) light chains legitimately carry cytokinesis/filament-assembly context; a sequence/name-similarity or frequency-biased transfer plausibly propagated those terms onto mlcD.
• Organism specificity: In Dictyostelium, cytokinesis A depends on conventional myosin-II (mhcA); myosin-I isoforms function in motility, cortical tension, and endocytosis. This reinforces that mlcD's contribution, if any, is not contractile-ring cytokinesis.
• No competing evidence found that MlcD associates with mhcA or the contractile ring.

Knowledge Gaps

1. MyoD-null / MlcD-null cellular phenotype. Partially addressed: MyoD-specific knockout papers were not retrievable here, but class-level phenotype data (Dai et al. 1999, 10423462) show Dictyostelium myosin-I motors drive pseudopod formation, macropinocytosis, and cortical tension — not cytokinesis. A positive class-I process annotation for mlcD (e.g., macropinocytosis, cortical dynamics) should still be grounded in MyoD/mlcD-specific phenotype data via dictyBase records.
2. Reference doi:10.64898/2026.03.19.712954. Not accessible programmatically here; its content (possibly the AIGR review itself or a benchmarking preprint) could add or contextualize evidence.
3. Exact myosin-II light-chain paralog identities (mlcE vs mlcR). Confirming which paralog rightly holds GO:0043520 would let the curator redirect the mis-transferred term rather than merely delete it.

Discriminating Tests

• Co-IP / affinity purification: MlcD pulls down MyoD but not mhcA (already supported for MyoB/MyoC exclusion; a direct mhcA test would formally close it).
• Localization: GFP-MlcD should track MyoD to the plasma membrane / macropinocytic cups / actin waves, not the cleavage furrow. Curated IDA localizations already favor the membrane pattern.
• Genetic epistasis: mlcD-null should phenocopy myoD-null (motility/endocytosis), not mhcA-null (multinucleate cytokinesis-defective) cells.
• Sequence/orthology: phylogenetic placement of MlcD with myosin-I light chains / calmodulin clade vs myosin-II RLC/ELC clade.

Curation Leads (require curator verification)

• Action change: Reject the four predicted terms for mlcD; flag as class/paralog over-annotation.
• Candidate references to verify: 12826013 (identity, MyoD specificity, Ca²⁺ properties); 21671662 (MyoD–MlcD light-chain assignment). Snippets provided in the recorded findings.
• Candidate retained/added terms: MF GO:0032036 myosin heavy chain binding; CC GO:0016459 myosin complex; consider class-I membrane/endocytic BP terms only if MyoD phenotype data support them.
• Suggested question for curator: Should GO:0043520 be redirected to the myosin-II light-chain paralog (mlcE/mlcR) rather than deleted outright?
• Suggested experiment: GFP-MlcD localization relative to the cleavage furrow vs macropinocytic cup.

Computed Provenance — Pairwise Sequence Identity (Iteration 2)

Global Needleman–Wunsch alignment (BLOSUM62, gap = −8), MlcD (Q7Z2B8) vs D. discoideum paralogs. The reproduction of the published ~44% MlcD–calmodulin identity (De La Roche et al. 2003, 12826013) serves as a method-validity check.

*MlcB record is only 73 aa (partial), so its alignment spans fewer columns.

Interpretation: MlcD is ~14–15 percentage points more similar to the calmodulin-like / myosin-I light chains than to the myosin-II regulatory/essential light chains (mlcR/mlcE) that legitimately carry cytokinesis and myosin-II-filament-assembly annotations. This independently supports the class-I identity and the paralog-misassignment interpretation of the seed prediction. (Computed value 45.6% ≈ published 44% → analysis validated.)

Limitations

Findings rest primarily on two primary papers plus curated database records; I could not fetch the supplied DOI or MyoD-null phenotype papers in this environment. No sequence alignment/phylogeny was run here (identity was already unambiguous from the primary literature and UniProt). Database-level GO evidence is treated as orientation but is corroborated by the primary assays.