MLO (barley, P93766) curation notes

2026-10-02 — initial review

Sources: UniProt P93766, GOA (2 IEA rows), falcon deep research, cached abstracts
(PMID:9054509, 10574976, 11919636, 12114562, 15703292, 15352871, 28095124) and full text
(PMID:20170486, 36588121). Most barley papers are cached as abstracts only.

Identity / structure

• Plant-specific 7-TM protein, N-terminus outside, C-terminus inside, plasma membrane
  PMID:10574976
  PMID:10574976
• Self-associates in planta PMID:15352871
• Biochemical activity unknown for barley MLO PMID:20170486

Molecular function evidence

• Ca2+-dependent calmodulin binding PMID:11919636
  and in vivo FRET PMID:15703292.
  Not in GOA -> proposed NEW GO:0005516.
• Channel hypothesis: Arabidopsis paralogs are Ca2+ channels
  PMID:36588121.
  Deep research cites a 2026 bioRxiv preprint (modeling) and reported Ba2+/Mg2+ currents for HvMLO, but no peer-reviewed barley conductance data in the cache. Not annotated; raised as a question.

Biological role

• Susceptibility factor: wild-type MLO enables fungal entry; loss gives resistance
  PMID:15703292.
• Negative regulation of defense / cell death
  PMID:12114562
  PMID:11919636.

Decisions

• GO:0006952 defense response (IEA, InterPro2GO): MODIFY -> GO:0031348 negative regulation
  of defense response. Sign of the IEA is wrong for the wild-type protein. Comparator:
  Arabidopsis MLO2 carries GO:0031348 (TAIR IMP, acts_upstream_of_or_within). No
  "negative regulation of defense response to fungus" term exists (checked QuickGO/OLS).
  Did NOT propose "defense response to fungus": that would assert MLO executes defense,
  which is a necessity/phenotype inversion.
• GO:0016020 membrane (IEA): MODIFY -> GO:0005886 plasma membrane.
• NEW GO:0005516 calmodulin binding (IDA, PMID:11919636).

Open questions

• Does HvMLO conduct Ca2+? If yes, calcium channel activity should become the core MF.
• Is the regulatory process term overreaching given purely genetic evidence?