| Feature (definition) | Evidence summary | Key quantitative details | Species context (P. putida vs P. aeruginosa vs P. fluorescens) | Key citation (with DOI URL and year) |
|---|---|---|---|---|
| Structure: AlgE is the alginate outer-membrane export porin | Primary structural studies identify AlgE as a monomeric 18-stranded antiparallel β-barrel outer-membrane protein required for alginate secretion; recent reviews retain this as the accepted model for Pseudomonas alginate export. | Narrow pore constriction reported at ~8 Å in the 2011 structure; later structural ensemble describes an electropositive pore of ~10 Å diameter depending on conformation. | Direct experimental structure/function: **P. aeruginosa**. Orthologous family/model support: **P. putida**. Export-factory context: **P. fluorescens**. | Whitney et al. 2011, PNAS, https://doi.org/10.1073/pnas.1104984108; Tan et al. 2014, https://doi.org/10.1107/S1399004714001850; Gheorghita et al. 2023, https://doi.org/10.1093/femsre/fuad060 (pqac-00000021, pqac-00000018, pqac-00000014) |
| Pore electrostatics: charge-based translocation path for an anionic polysaccharide | The AlgE lumen is highly electropositive, supporting selective passage of negatively charged alginate; mutagenesis of basic constriction residues impairs secretion, consistent with charge complementarity rather than nonspecific diffusion. | Constriction residues highlighted include Lys47, Arg74, Arg129, Arg152, Arg353, Arg362, Arg459; triple mutants such as K47E/R353A/R459E and R74E/R362A/R459E strongly reduce alginate production. | Strongest mutational evidence: **P. aeruginosa**. Family/domain inference for Q88NC8 in **P. putida** is consistent with same mechanism. | Whitney et al. 2011, https://doi.org/10.1073/pnas.1104984108; Rehman & Rehm 2013, https://doi.org/10.1128/AEM.03960-12 (pqac-00000021, pqac-00000016) |
| Gating loops L2/T8: extracellular and periplasmic gates controlling pore opening | Structural and MD studies support a double-gate model in which extracellular loop L2 and periplasmic loop T8 occlude or permit passage; T8 is especially implicated in conductance control, and AlgK is proposed to help open the periplasmic gate during secretion. | Deleting T8 caused ~3-fold higher iodide efflux in liposome assays; ΔT8 and ΔL2 complemented alginate secretion only partially, restoring ~48% and ~60% of WT production, respectively. | Direct assays: **P. aeruginosa**. Mechanistic model applied to orthologous **P. putida** AlgE in later structural modeling. | Whitney et al. 2011, https://doi.org/10.1073/pnas.1104984108; Tan et al. 2014, https://doi.org/10.1107/S1399004714001850; Gheorghita et al. 2023, https://doi.org/10.1093/femsre/fuad060 (pqac-00000021, pqac-00000020, pqac-00000018, pqac-00000014) |
| Substrate specificity assay: recognition of alginate-like uronate substrate | Reconstituted AlgE channels are sensitive to alginate-related substrate analogs; di-mannuronic acid inhibits halide efflux, supporting specificity for polymannuronate/alginate rather than a generic porin function. Citrate bound in structural work was proposed to mimic alginate uronate units in the pore. | Excess di-mannuronic acid (MM) reduced iodide efflux to near empty-liposome levels; one assay used a 100:1 MM excess, another 1 mM MM. | Functional specificity shown in **P. aeruginosa** AlgE; relevant to **P. putida** Q88NC8 because the same AlgE family/domain architecture is modeled for the ortholog. | Whitney et al. 2011, https://doi.org/10.1073/pnas.1104984108; Tan et al. 2014, https://doi.org/10.1107/S1399004714001850 (pqac-00000020, pqac-00000018) |
| Interaction/complex role: AlgE as part of a trans-envelope Alg8/Alg44/AlgX/AlgK/AlgE system | Genetic and structural studies support AlgE as the outer-membrane exit pore within a larger secretion/modification complex. Loss of algE destabilizes AlgK/AlgX and lowers Alg44 abundance; recent work places AlgE with AlgK and AlgX in an AlgEKX outer-membrane secretion/modification complex. | Deleting algE abolishes polymer secretion and leads to release of free uronic acids/degradation products instead of intact alginate; periplasmic turn 4 contributes to complex stability. | Core evidence from **P. aeruginosa**; recent structural models include **P. putida** AlgE/AlgK in the complex. | Rehman & Rehm 2013, https://doi.org/10.1128/AEM.03960-12; Moradali et al. 2015, https://doi.org/10.1128/mBio.00453-15; Gheorghita et al. 2022, https://doi.org/10.1038/s41467-022-35131-6; Gheorghita et al. 2023, https://doi.org/10.1093/femsre/fuad060 (pqac-00000016, pqac-00000005, pqac-00000003, pqac-00000014) |
| P. putida-specific relevance: UniProt Q88NC8/PP_1284 assignment supported by orthology and structural modeling | Retrieved literature does not explicitly cite Q88NC8 or PP_1284, but a high-impact 2022 study explicitly models **P. putida** AlgE (AlgE_Pp) in the AlgEK and AlgEKX assemblies and compares it with the experimentally solved **P. aeruginosa** AlgE structure, supporting annotation of Q88NC8 as an AlgE-family alginate export porin. | Evidence is indirect for accession-level mapping; no retrieved paper directly reports Q88NC8-specific biochemical assays. | **P. putida**: modeled ortholog in AlgEKX. **P. aeruginosa**: direct structure/function benchmark. | Gheorghita et al. 2022, https://doi.org/10.1038/s41467-022-35131-6 (pqac-00000000, pqac-00000005) |
| Cellular localization: outer membrane, periplasm-facing interface | Topology mapping and structural work place AlgE in the outer membrane with both termini periplasmic; extracellular loops and periplasmic turns are essential for folding, gating, and interaction with the periplasmic secretion scaffold. | AlgE is ~54 kDa; topology studies support 9 extracellular loops and 8 periplasmic turns in an 18-stranded barrel. | Direct topology/mutagenesis: **P. aeruginosa**; family-level inference for **P. putida**. | Hay et al. 2010, https://doi.org/10.1128/AEM.02945-09; Whitney et al. 2011, https://doi.org/10.1073/pnas.1104984108 (pqac-00000019, pqac-00000021) |
| Application/industrial relevance: biofilm and fermentation engineering in Pseudomonas | While no 2023–2024 study in the retrieved set measured AlgE-specific production yields in **P. putida** KT2440, recent engineering of alginate-related biofilm traits in P. putida shows practical fermentation relevance: reducing alginate-associated biofilm formation can improve bioreactor handling. | In a 2024 fermentation study, a P. putida triple mutant including ΔalgA showed 20–30% less biofilm at 24 h and 33.3%/40% less biofilm at 48/72 h; final OD600 remained similar to WT (1.98 ± 0.03 vs 2.005 ± 0.02). | Direct application data: **P. putida**. This is pathway-adjacent rather than AlgE-specific. | Frolov et al. 2024, https://doi.org/10.3390/fermentation10120606 (pqac-00000022) |
| Application/process biology: visualization of alginate “factories” and bottlenecks in production | Immunogold TEM studies in nonpathogenic Pseudomonas show AlgE-containing export complexes cluster in the outer membrane and depend on scaffold components for localization, informing process engineering of microbial alginate production. Factory abundance did not scale simply with production level, implying metabolic precursor/energy supply can be more limiting than exporter count. | No correlation between number of detectable factories and alginate production level; algC deficiency did not reduce factory numbers. | Direct visualization and process insight: **P. fluorescens**; useful comparator for **P. putida** cell-factory engineering. | Maleki et al. 2016, https://doi.org/10.1128/AEM.03114-15; Maleki et al. 2017, https://doi.org/10.1016/j.nbt.2016.08.005 (pqac-00000023, pqac-00000024) |


*Table: This table summarizes the strongest structural, mechanistic, and application-relevant evidence for AlgE as an alginate export porin, with emphasis on how direct P. aeruginosa experiments support annotation of the P. putida KT2440 Q88NC8 ortholog. It also highlights where evidence is direct, modeled, or still limited for the specific accession.*