Reflectin 2b (Q6WDN4) - Euprymna scolopes - Review Notes

Initial assessment (2026-05-02)

Key facts

• UniProt: Q6WDN4 (unreviewed TrEMBL entry), 284 AA, 37.2 kDa
• No formal gene symbol in UniProt; submitted name "Reflectin 2b"
• Genomic DNA sequence from GenBank AY294653
• Originally characterized in Crookes et al. 2004 (Science) PMID:14716016
• Protein evidence level: PE4 (Predicted) - only nucleotide sequence evidence

Unique features of reflectin proteins

• Amino acid composition is highly unusual: ~57% Tyr, Met, Arg, Trp; completely lacking Ala, Ile, Leu, Lys PMID:14716016
• Five conserved repeating domains; at least six genes in three subfamilies in E. scolopes PMID:14716016
• No homologs outside Cephalopoda - truly lineage-specific PMID:14716016
• Intrinsically disordered; undergoes liquid-liquid phase separation PMID:31558609

Annotation status

• ZERO GO annotations in QuickGO/GOA for Q6WDN4 as of 2026-05-02
• This is an annotation gap for one of the most studied cephalopod proteins
• Proposed NEW annotations: GO:0005198 (structural molecule activity), GO:0140693 (molecular condensate scaffold activity), GO:0051260 (protein homooligomerization), GO:0043473 (pigmentation), GO:0043698 (iridosome), GO:0065003 (protein-containing complex assembly)

Static vs. dynamic iridescence

• In E. scolopes: light-organ iridescence is STATIC (constitutive) PMID:19776150
• In loliginid squid (Doryteuthis/Loligo): dermis has DYNAMIC tunable iridescence controlled by ACh/muscarinic cholinergic system [PMID:19776150, PMID:22896651]
• Dynamic tunability is correlated with specific reflectin subtypes (A1, A2) NOT found in E. scolopes [PMID:19776150, PMID:25918159]
• Key question: which reflectin subtypes confer tunability vs. static reflectance?

Evolutionary origin

• Guan et al. 2017 PMID:28889973 traced reflectin origin to a transposon from Vibrio fischeri (Aliivibrio fischeri)
• Core repeating octapeptide (protopeptide) shared between reflectin and bacterial transposase
• This is a remarkable case of horizontal gene transfer from a symbiont contributing a novel protein family
• The V. fischeri symbiosis with E. scolopes light organ adds an intriguing dimension

Phosphorylation mechanism (from loliginid studies)

• ACh activates muscarinic receptors -> signal transduction -> tyrosine kinase phosphorylation of reflectins PMID:19776150
• Phosphorylation neutralizes cationic linker charges -> overcomes coulombic repulsion -> condensation + assembly PMID:31558609
• Assembly proceeds through dynamically arrested LLPS intermediate PMID:31558609
• Precise relationship: net charge density -> assembly size -> platelet dimensions -> reflected color PMID:31558609
• Genistein (tyrosine kinase inhibitor) blocks both phosphorylation and iridescence PMID:19776150

Proposed new GO term

• "structural coloration" as a child of GO:0043473 (pigmentation) would be valuable
• Would serve reflectins, bird structural coloration (collagen arrays), butterfly photonic crystals, fish guanine crystals