The IgStrand numbering scheme is the universal residue numbering for the Ig-fold, introduced by Tawfeeq et al., PLOS Comput Biol 2025 (doi:10.1371/journal.pcbi.1012813).
This document is a short reference for the encoding used in IgFold2D templates and assignments. See the paper for the full structural rationale.
A 4-digit code ijxx:
i (thousands) — strand identityj (hundreds) — strand variant (insertion / split)xx (tens, ones) — position within the strand, centered on 50 (the structural anchor)i)i |
Strand | Sheet |
|---|---|---|
| 1 | A | 1 (ABED) |
| 2 | B | 1 |
| 3 | C | 2 (A’GFCC’C”) |
| 4 | C’ | 2 |
| 5 | C” | 2 |
| 6 | D | 1 |
| 7 | E | 1 |
| 8 | F | 2 |
| 9 | G | 2 |
j)For canonical strands: j = 5. The anchor position of each strand is i5 50:
| Anchor | Strand | Conserved residue type (in IgV) |
|---|---|---|
| 1550 | A | E / Q |
| 1850 | A’ | hydrophobic (special: j=8) |
| 2550 | B | C (disulfide) |
| 3550 | C | W |
| 4550 | C’ | L |
| 5550 | C” | varied |
| 6550 | D | varied |
| 7550 | E | L |
| 8550 | F | C (disulfide) |
| 9550 | G | varied |
For insertions (non-canonical strands), j deviates from 5:
j |
Position relative to canonical strand |
|---|---|
| 8 | A’ (split A, second half on sheet 2) — unique exception |
| 6 | First insertion after the canonical strand (e.g., G+ = 96xx) |
| 7 | Second insertion after (e.g., G++ = 97xx) |
| 4 | First insertion before (e.g., A− = 14xx) |
| 3 | Second insertion before (e.g., A−− = 13xx) |
xx)Centered on 50 (the anchor). Residues N-terminal to the anchor get 49, 48, 47, … ; C-terminal get 51, 52, 53, …. Loop residues continue the numbering of the flanking strand (variable per domain — see topology-rules.md §3).
A strand of 7 residues might span ij47 to ij53 around the anchor ij50.
| Code | Meaning |
|---|---|
| 1550 | A strand anchor |
| 1849 | A’ strand, one residue N-terminal to A’ anchor |
| 1851 | A’ strand, one residue C-terminal to A’ anchor |
| 2553 | B strand, 3 residues C-terminal to B anchor (still in strand) |
| 9650 | G+ strand anchor (insertion after G, e.g. in CD3γ) |
| 1450 | A− strand anchor (insertion before A) |
Anchors at xx50 are structurally conserved positions — defined by backbone hydrogen-bond network across the β-sandwich, not by sequence conservation. They are:
In IgFold2D, anchor cells carry the flag is_anchor = true in the registry.
Tawfeeq M, Lange J, Tcherepanova I, Hassan R, Lupyan D, Sengupta D, Cachau RE, Olson AJ, Youkharibache P (2025) A universal residue numbering scheme for the Immunoglobulin-fold (Ig-fold). PLOS Comput Biol 21(4): e1012813. doi:10.1371/journal.pcbi.1012813