The organizing unit of GPCR activation-switch architecture: a phylogenetic-scale census

The intracellular "ionic lock" between Arg3.50 of the D(E)RY motif and an acidic residue at position 6.30 is a textbook feature of class A G-protein-coupled receptor (GPCR) activation. Yet across a 11,609-sequence cross-species survey, the complete classic lock (a charged residue at 6.30 paired with the DRY arginine) is present in only about a quarter of receptors — consistent with prior single-statistic reports that ~ 34% of class A receptors carry both partners (Kling et al. 2016) and that the 6.30 acidic residue is conserved in only 25–31% of sequences (Hauser et al. 2021). This raises a question those reports did not address: when the classic lock is absent, what stabilizes the conserved DRY arginine, and is the answer organized by the receptor's ligand? We classify Arg3.50-protection mechanisms genome-wide into six categories and show that ligand family does not predict which mechanism a receptor uses — the same mechanism recurs across aminergic, peptide, and nucleotide families, while the true organizing unit descends to the level of individual genes and lineages. We then show that the three components most implicated in the activation switch — the 6.30 lock, Tyr5.58, and the sodium-pocket residues — coevolve as a coupled network after phylogenetic correction (Pagel pairwise tests all p < 10⁻¹⁷; a joint eight-state corHMM model favouring full coupling over independence by ΔAICc = 439), a signal robust to parametric bootstrap and to removal of the largest taxonomic groups. Finally, using generic-number-resolved structural analysis of experimentally determined structures, we document a specific alternative solution — an acidic residue within TM6 that stabilizes Arg6.30 — in five receptors with solved structures (PAR2, CCR5, CCR2, AGTR2, EDNRA). Together these results argue that activation-switch architecture is organized not by ligand identity but by a coevolving structural network, realized through several graded local solutions.