CRT1 (then called CR518) was identified as a cephalopod-specific chemotactile receptor enriched in octopus sucker sensory epithelium.

CRT1 is activated by poorly soluble terpenes (polygodial, atractylon) and prey extract.

CRs form homomeric and heteromeric pentameric ion channel complexes permeable to Na+, K+, Cs+, and Ca2+.

CRs are co-expressed in combinatorial patterns in chemosensory cells.

Terpene agonists modify chemotactile exploratory behavior in a chemical-specific manner.

Atractylon at approximately 30 uM elicited significant inward currents in CRT1-expressing cells. CRT1 also responds to nootkatone (a sesquiterpene), and chloroquine (25 uM).

CRT1/CR828 heteropentameric receptors showed modified ligand sensitivities (enhanced nootkatone response at lower concentrations) and significantly enhanced Ca2+ permeability compared to CRT1 homomers (p < 0.0001).

Octopus ink extract suppresses CRT1-mediated chemotactile responses, inhibiting terpene-evoked currents. This suggests ink release during escape responses masks food-associated chemical cues, prioritizing flight over foraging.

CRT1 specifically responds to hydrophobic terpenes while being insensitive to bitter compounds (denatonium), in contrast to squid CRB1.

CRs evolved from ancestral nicotinic acetylcholine receptors through gene duplication and structural diversification of the ligand-binding site.

CR agonists elicit robust axial nerve activity and autonomous arm movement.

CRs are cephalopod-specific, absent in nautiloids and other molluscs.

The CRT subfamily is present in both octopus and squid lineages.

Cryo-EM structure of CRT1 at 2.62 A resolution (PDB 8EIS) reveals homopentameric architecture with hydrophobic ligand-binding pocket.

The ligand-binding site is subject to diversifying selection, mediating detection of new molecules versus ancestral neurotransmitters.

E104 residue in the vestibule is critical for Ca2+ permeability, analogous to alpha7 nAChR E97.

CRT1 localizes to apical dendritic endings of sucker receptor cells.

CRT1 agonists elicit autonomous arm behavior and axial nerve activity.

In O. bimaculoides, 26 intronless CR genes are arranged in tandem on chromosome 15, suggesting origin through retrotransposition from an ancestral nAChR gene followed by rapid tandem duplication.

CR sequences show accelerated evolution compared to nAChRs (LRT p < 0.001), with diversifying selection concentrated at the ligand-binding interface. Key amino acids in the orthosteric binding domain show signatures of positive selection across octopus species.

CRT1 possesses an additional disulfide bond (between loops beta4 and beta7 of the ECD) not found in canonical Cys-loop receptors. This extra bond increases rigidity of the extracellular domain and is essential for channel function (Cys-to-Ala mutations abolish activity). CRT1 has 3 disulfide bonds per ECD subunit vs 2 in typical Cys-loop receptors.

The ligand-binding pocket of CRT1 is significantly larger and more hydrophobic than that of the alpha7 nAChR, enabling binding of bulky, poorly water-soluble terpene ligands. Hydrophobic detergent molecules were observed in the pocket during structure determination, illustrating the site's affinity for greasy compounds.

CRT1 is activated by harmine-3-carboxylic acid from crab-associated bacteria and lumichrome from egg-associated bacteria.

Different microbial signals bind CRT1 in distinct structural conformations.

Microbial signals drive specific predatory versus maternal behaviors through CRT1.

S189 mutation impairs activation by beta-carboline alkaloids.

Cryo-EM structures in complex with agonists (PDB 9E6B, 9E6C, 9E6D).

Harmine-3-carboxylic acid, produced by bacteria associated with crab prey, specifically activates CRT1 to drive predatory behavior. Lumichrome, a flavin produced by bacteria on octopus eggs, activates CRT1 to modulate maternal care behavior.

One and the same CRT1 receptor can respond to multiple different microbial metabolite ligands and produce distinct ionic currents depending on the bound molecule, potentially enabling discrimination of different ecological contexts through a single receptor.

CRs are the most derived lineage of acetylcholine-like receptors within mollusks, representing an early cephalopod-specific innovation.

All CR genes are intronless, arising as retrocopies.

O. bimaculoides possesses over twenty different CR genes.

CRs mediate sensation of diverse chemical cues including terpenes, hormones, and bile acids.

CRs are not found outside of coleoid cephalopod lineages.

Homopentameric assembly with 5-fold symmetric ECD and TMD; M1-M3 helices resolved, M4 disordered (absent density, predicted by AlphaFold)

Loop C is shortened by 6 residues compared to alpha7 nAChR, contributing to a flatter, more open binding pocket that cannot accommodate small polar ligands like ACh

Ligand binding dominated by the complementary subunit face, opposite to canonical nAChRs where the principal subunit contributes most contacts

Three disulfide bonds per subunit ECD (vs two in typical Cys-loop receptors): the canonical Cys-loop (beta6-beta7), the vicinal pair (Loop C), and a novel beta4-beta7 bond that rigidifies the binding pocket. Disrupting the novel disulfide abolishes function

E104 forms an anionic ring at the vestibule entrance controlling Ca2+ selectivity; E104A and E104K mutations dramatically reduce Ca2+ permeability (p<0.001) without affecting monovalent cation permeation

Binding-site mutations at diversifying-selection residues produce constitutive channel activity with reduced ligand sensitivity, demonstrating these residues gate the transition between resting and active states

Diosgenin (GDN) visualized bound in the orthosteric pocket at the subunit interface, with contacts primarily to hydrophobic and aromatic residues on the complementary face