## Table 1 (revised). Structural evidence for candidate intra-TM6 acidic partners to Arg6.30

| Receptor | PDB | Acidic partner | Generic position | Min. heavy-atom dist. (Å) | Salt-bridge range? | Resolution | Method | Engineered mutations | Construct notes |
|---|---|---|---|---|---|---|---|---|---|
| PAR2 | 5NDD | Glu279 | 6.27x28 | 3.21 (OE2–NH1) | Yes (<3.5 Å) | 2.80 Å | X-ray | 10 | Thermostabilized (StaR) construct, antagonist-bound (AZ8838); not a native resting state |
| CCR5 | 5UIW | Glu227 | 6.27x27 | not assessable — carboxylate (OE1/OE2) unresolved; CB–Arg230 = 5.51 Å | Cannot assess | 2.20 Å | X-ray | 4 | Side-chain density incomplete beyond Cβ in this structure |
| CCR2 | 7XA3 | Glu235 | TM6 (6.28 region) | 4.16 (OE1–NH2, partner is Arg238) | Borderline (<4.5 Å) | 2.90 Å | cryo-EM | 0 (tags only) | CCL2–CCR2–Gi **active-state** complex, not inactive/resting |
| AGTR2 | 5UNF | Asp252 | 6.31x31 | 3.23 (OD1–NE) | Yes (<3.5 Å), closest of the set | 2.80 Å | X-ray | 6 | BRIL fusion chimera + engineered mutations; not wild-type |
| EDNRA (inactive) | 8XVJ | Glu303 | 6.32x32 | 9.16 (NE–OE2) | No — far exceeds bonding distance | 3.26 Å | cryo-EM | 0 (tags only) | DRY-motif Asp182 farther still (9.03 Å); neither partner within salt-bridge range |
| EDNRA (active) | 8HCQ | Glu296 | 6.25x25 | not assessable — carboxylate unresolved; CB–Arg301 = 9.59 Å | Cannot assess | 3.01 Å | cryo-EM | 0 (tags only) | Side-chain density incomplete beyond Cβ; CB-level distance regardless far exceeds bonding range |

**Note on interpretation.** Of the five receptors, only PAR2 and AGTR2 show a genuine
salt-bridge-range distance (<3.5 Å) between resolved carboxylate and guanidinium heavy
atoms — and both of these structures are engineered constructs (thermostabilized/fusion
chimera) rather than unmodified wild-type receptors, so a native inactive-state distance
cannot be independently confirmed from these coordinates alone. CCR2 is borderline (4.16 Å)
in an active-state complex. EDNRA shows no salt-bridge-range contact in either resolved
conformational state. Two structures (CCR5, EDNRA-active) have carboxylate side chains
unresolved in the deposited electron density/map, making a heavy-atom distance impossible
to compute; the previously reported centroid distances for these cases were necessarily
based on incomplete side-chain models (Cβ-level proxies) and should not be read as validated
salt-bridge measurements.

Given this evidence pattern, we describe the TM6-internal acidic residue as a **candidate
intra-TM6 proximity/configuration** rather than a demonstrated stabilizing or rescue
interaction — a claim of "stabilization" would require site-directed mutagenesis or
molecular dynamics evidence that is not available for any of these five cases (see Discussion,
Limitations).