S-layer

Deep research

Generated by just research-trait; source: research/traits/morphology/s_layer-deep-research-falcon.md

# S-layer Trait Causal Graph: Curation-Focused Research Report

## 1. Trait Scope Summary

The S-layer (surface layer) is a morphology trait defined as a crystalline, two-dimensional array of self-assembling proteinaceous (glyco)protein subunits coating the cell surface of many bacteria and most archaea (METPO identifier: traitmech:000064) (pum2013slayerproteinselfassembly pages 1-4, sleytr2025slayersfroma pages 2-4). S-layers represent one of the most abundant cellular protein structures on Earth and the simplest biological membrane developed during evolution (sleytr2025slayersfroma pages 2-4). They exhibit oblique (p1, p2), square (p4), or hexagonal (p3, p6) lattice symmetries with unit cell dimensions of 3–30 nm, thicknesses of 5–20 nm in bacteria (up to ~70 nm in archaea), and 30–70% porosity with uniform pores of 2–8 nm (pum2013slayerproteinselfassembly pages 1-4, sleytr2025slayersfroma pages 2-4). Hexagonal symmetry predominates among archaea while varied symmetries are found in bacteria (sleytr2025slayersfroma pages 2-4, pum2021patternsinnature—slayer pages 6-8).

**Boundary cases and distinctions from nearby traits:**
- S-layers are distinct from sheaths (e.g., *Methanospirillum hungatei*), which are non-crystalline amyloid-like protein assemblies, though both are cell-surface proteinaceous structures.
- S-layers differ from peptidoglycan cell walls, outer membranes, and capsules in being exclusively composed of (glyco)protein subunits arranged in a periodic lattice. In archaea that possess S-layers as the sole cell wall component, they functionally replace peptidoglycan (grillwalcher2025anewage pages 1-2).
- The trait is defined at the CLASS level and encompasses diverse S-layer protein families (SLH-domain containing, CWB2-domain containing, non-SLH lactobacilli types, two-component archaeal types) that converge on the same cell-surface phenotype.

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## 2. Candidate Causal Graph Nodes

The following table organizes all candidate nodes by type, with ontology groundings where available:

| Node Name | Node Type | Suggested CURIE | Description |
|---|---|---|---|
| **PROTEINS/GENES** ||||
| SlpA | protein/gene | label-only candidate | *Clostridioides difficile* S-layer precursor protein; exported via accessory Sec, proteolytically cleaved into HMW/LMW SLPs, then assembled into the mature S-layer (kirk2017characteristicsofthe pages 4-5) |
| SlaA | protein/gene | label-only candidate | Outer extracellular S-layer glycoprotein of *Sulfolobus acidocaldarius*; highly glycosylated and forms the main porous lattice (gambelli2024structureofthe pages 10-12, gambelli2024structureofthe pages 2-3) |
| SlaB | protein/gene | label-only candidate | Inner/membrane-bound S-layer component of *S. acidocaldarius*; trimeric anchor linking SlaA lattice to the membrane (gambelli2024structureofthe pages 10-12, gambelli2024structureofthe pages 2-3) |
| EA1 | protein/gene | label-only candidate | *Bacillus anthracis* stationary-phase S-layer protein; calcium-dependent assembly protein with N-terminal SLH domains and six Ig-like assembly domains (sogues2023structureandfunction pages 1-2, sogues2023structureandfunction pages 2-3) |
| Sap | protein/gene | label-only candidate | *B. anthracis* exponential-phase S-layer protein; mutually exclusive with EA1 and anchored to SCWP through SLH domains (sogues2023structureandfunction pages 1-2, sogues2023structureandfunction pages 7-8) |
| Cwp84 | protein/gene | label-only candidate | Cell wall-localized cysteine protease that cleaves SlpA precursor into mature HMW and LMW S-layer proteins in *C. difficile* (kirk2017characteristicsofthe pages 4-5) |
| Cwp66 | protein/gene | label-only candidate | *C. difficile* cell wall protein and adhesin contributing to adhesion, stress tolerance, and antibiotic resistance (chandra2023hostimmuneresponses pages 4-6) |
| Cwp2 | protein/gene | label-only candidate | Major *C. difficile* cell wall protein associated with the S-layer and implicated in adhesion/colonization (chandra2023hostimmuneresponses pages 4-6) |
| RsaA | protein/gene | label-only candidate | Major *Caulobacter crescentus* S-layer protein inserted at poles and mid-cell during cell-cycle-coordinated S-layer biogenesis (herdman2023cellcycledependent pages 8-11, herdman2023cellcycledependent pages 11-15) |
| SlpB | protein/gene | label-only candidate | Lactobacillus S-layer protein paralog/accessory component; present in S-layer loci but often silenced in studied strains (sagmeister2024themoleculararchitecture pages 1-2, hynonen2013lactobacillussurfacelayer pages 7-8) |
| SlpX | protein/gene | label-only candidate | Lactobacillus accessory S-layer protein incorporated into the lattice, especially under environmental stress; linked to cell integrity (sagmeister2024themoleculararchitecture pages 1-2, sagmeister2024themoleculararchitecture pages 9-9) |
| SecA2 | protein/gene | label-only candidate | Accessory Sec ATPase in the *C. difficile* S-layer locus required for export of SlpA and related S-layer proteins (kirk2017characteristicsofthe pages 4-5, barwinskasendra2025evolutionaryplasticityof pages 21-21) |
| MreB | protein/gene | UniProtKB:P0A9X4 | Bacterial actin homolog; required for spatial coordination of S-layer insertion with elongation zones in *C. crescentus* (herdman2023cellcycledependent pages 5-8, herdman2024cellcycledependent pages 4-5) |
| Saci1846 | protein/gene | label-only candidate | Thermopsin-like protease implicated with SlaB in anchoring/assembly of the *Sulfolobus* S-layer; taxon-specific and still emerging evidence (foo2025themechanicsof pages 23-26) |
| TfsA | protein/gene | label-only candidate | *Tannerella forsythia* O-glycosylated S-layer protein secreted by T9SS and assembled into the cell-surface lattice (paillat2023ajourneywith pages 8-9) |
| TfsB | protein/gene | label-only candidate | *T. forsythia* O-glycosylated S-layer protein secreted by T9SS and assembled with TfsA into the S-layer (paillat2023ajourneywith pages 8-9) |
| AglB | protein/gene | label-only candidate | Archaeal oligosaccharyltransferase catalyzing the final step of N-glycosylation; essential in *S. acidocaldarius* (gambelli2024structureofthe pages 12-13) |
| **DOMAINS/MOTIFS** ||||
| SLH domain | domain/motif | pfam:PF00395 | S-layer homology domain mediating non-covalent anchoring of Bacillaceae S-layer proteins to secondary cell wall polysaccharides (pum2013slayerproteinselfassembly pages 1-4, sogues2023structureandfunction pages 1-2) |
| CWB2 domain | domain/motif | pfam:PF04122 | Cell wall binding 2 motif in *C. difficile* HMW SLP region; mediates anchoring to PS-II (kirk2017characteristicsofthe pages 4-5) |
| signal peptide | domain/motif | GO:0005048 | N-terminal secretion signal directing S-layer proteins into export pathways such as Sec or accessory SecA2 (kirk2017characteristicsofthe pages 4-5, paillat2023ajourneywith pages 1-3) |
| Ig-like domain | domain/motif | label-only candidate | Immunoglobulin-like assembly domain forming the tile-like assembly regions of EA1 and Sap (sogues2023structureandfunction pages 1-2, sogues2023structureandfunction pages 7-8) |
| TAB domain | domain/motif | label-only candidate | Teichoic-acid-binding domain of Lactobacillus SlpA proteins implicated in LTA/WTA attachment (sagmeister2024themoleculararchitecture pages 9-9, sagmeister2024themoleculararchitecture pages 1-2) |
| **CHEMICALS/METABOLITES** ||||
| Ca2+ ions | chemical | CHEBI:29108 | Divalent cation commonly required for S-layer reassembly and, in EA1 and archaeal systems, structural stabilization/assembly control (sleytr2025slayersfroma pages 19-20, sogues2023structureandfunction pages 2-3) |
| secondary cell wall polymer (SCWP) | chemical/polymer | label-only candidate | Bacillaceae wall polymer recognized by SLH domains to anchor S-layer proteins such as EA1 and Sap (sogues2023structureandfunction pages 1-2, sogues2023structureandfunction pages 7-8) |
| polysaccharide II (PS-II) | chemical/polymer | label-only candidate | *C. difficile* anionic wall polymer bound by CWB2 motifs to attach SlpA-derived heterodimers to the cell wall (kirk2017characteristicsofthe pages 4-5) |
| lipoteichoic acid (LTA) | chemical/polymer | CHEBI:24402 | Lactobacillus cell wall polymer serving as S-layer attachment ligand for TAB-containing SlpA proteins (sagmeister2024themoleculararchitecture pages 1-2, sagmeister2024themoleculararchitecture pages 9-9) |
| wall teichoic acid (WTA) | chemical/polymer | CHEBI:7744 | Cell wall polymer implicated in Lactobacillus S-layer attachment alongside LTA (sagmeister2024themoleculararchitecture pages 1-2) |
| N-glycans | chemical/glycan | CHEBI:50699 | N-linked glycans decorating archaeal S-layer proteins such as SlaA/SlaB; associated with thermostability and surface properties (gambelli2024structureofthe pages 1-2, gambelli2024structureofthe pages 12-13) |
| O-glycans | chemical/glycan | label-only candidate | O-linked glycans decorating some bacterial S-layer proteins, including *T. forsythia* TfsA/TfsB (paillat2023ajourneywith pages 8-9) |
| peptidoglycan | chemical/polymer | CHEBI:52722 | Structural cell wall polymer beneath many bacterial S-layers; S-layer insertion coordinates with zones of peptidoglycan turnover (herdman2024cellcycledependent pages 1-2, herdman2023cellcycledependent pages 8-11) |
| **SECRETION SYSTEMS** ||||
| Sec pathway | secretion system | GO:0015031 | General secretion pathway exporting signal-peptide-containing proteins across the cytoplasmic membrane; used upstream of several S-layer systems (kirk2017characteristicsofthe pages 4-5, paillat2023ajourneywith pages 1-3) |
| accessory Sec (SecA2) | secretion system | label-only candidate | Specialized Sec branch in *C. difficile* associated with S-layer protein export, especially SlpA secretion (kirk2017characteristicsofthe pages 4-5) |
| Type IX secretion system (T9SS) | secretion system | label-only candidate | Bacteroidota secretion machine exporting S-layer glycoproteins such as TfsA/TfsB after Sec-dependent periplasmic transit (paillat2023ajourneywith pages 8-9, paillat2023ajourneywith pages 1-3) |
| signal peptide-dependent export | secretion process | GO:0006614 | Protein export logic in which N-terminal signal peptides target S-layer precursors to translocation pathways (kirk2017characteristicsofthe pages 4-5, paillat2023ajourneywith pages 1-3) |
| **BIOLOGICAL PROCESSES** ||||
| self-assembly | biological process | GO:0043934 | Intrinsic spontaneous assembly of S-layer proteins into ordered arrays on cell surfaces or in vitro (pum2021patternsinnature—slayer pages 2-4, pum2013slayerproteinselfassembly pages 4-6) |
| 2D crystalline lattice formation | biological process | label-only candidate | Formation of porous para/crystalline monolayers characteristic of S-layers (pum2013slayerproteinselfassembly pages 1-4, gambelli2024structureofthe pages 2-3) |

Showing the first 60 of 292 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.