# Nodulin
**Nodulin** is the general name for a nodule-specific plant protein, and in its best-studied form, **nodulin 26** (NOD26), a 26.5 kDa transmembrane aquaporin of the soybean (*Glycine max*) root nodule symbiosome membrane that transports water, glycerol, formamide, and ammonia between the host plant and its nitrogen-fixing bacterial symbionts[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC340469/) • [2](https://pubmed.ncbi.nlm.nih.gov/9195927/). Nodulins are host proteins whose genes are expressed specifically or predominantly in legume root nodules, the organs that house rhizobial bacteria during symbiotic nitrogen fixation[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC340469/). Nodulin 26 is the archetype of the nodulin 26-like intrinsic protein (NIP) family, a group of multifunctional major intrinsic proteins unique to plants[3](https://www.sciencedirect.com/science/article/pii/S0005273606001118).
| Key fact | Detail |
|---|---|
| Identity | 26.5 kDa transmembrane polypeptide of the soybean peribacteroid (symbiosome) membrane, cloned in 1987 by Fortin, Morrison, and Verma[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC340469/) |
| Abundance | Estimated at 10% of the protein content of the symbiosome membrane; widely used as a marker for that membrane[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC4266029/) |
| Transport | Water, glycerol, formamide, and ammonia; symbiosome membrane osmotic water permeability of 0.05 ± 0.003 cm/s at 20 °C[2](https://pubmed.ncbi.nlm.nih.gov/9195927/) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC4266029/) |
| Channel conductance | Single-channel water conductance of 3.2 ± 1.3 × 10−15 cm³/s, about 30-fold lower than the red blood cell water channel AQP1[2](https://pubmed.ncbi.nlm.nih.gov/9195927/) |
| Ion channel activity | Maximum unitary conductance of 3.1 nS in 1 M KCl, weakly selective for anions and voltage-sensitive[5](https://pubmed.ncbi.nlm.nih.gov/7517934/) |
| Regulation | Phosphorylation at Ser-262 by a calcium-dependent protein kinase stimulates water channel activity[3](https://www.sciencedirect.com/science/article/pii/S0005273606001118) • [6](https://pmc.ncbi.nlm.nih.gov/articles/PMC8921385/) |
| Family | Archetype of the NIP (nodulin 26-like intrinsic protein) aquaglyceroporin subfamily, unique to plants[3](https://www.sciencedirect.com/science/article/pii/S0005273606001118) |
## Discovery
Nodulin 26 was identified in 1987 by M. G. Fortin, N. A. Morrison, and [D. P. S. Verma](https://www.edgepedia.org/desh-pal-s-verma), who used antibodies raised against purified soybean peribacteroid membrane (the membrane enclosing rhizobial bacteroids, also called the symbiosome membrane) to immunoprecipitate polysomes and screen a nodule-specific cDNA library. Clone p1B1 encoded the 26.5 kDa polypeptide, which the anti-membrane serum specifically precipitated[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC340469/).
The same study distinguished nodulin 26 from nodulin 24, another peribacteroid membrane nodulin: nodulin 26 has the features of a transmembrane protein, while nodulin 24 appears to be a surface protein of the membrane. Their expression patterns also differ. Nodules induced by *Bradyrhizobium japonicum* mutants that arrest development before bacteria are released from infection threads express nodulin 24 only at a very low level, whereas nodulin 26 expression occurs fully in nodules that form infection threads alone and is not affected by the release of bacteria[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC340469/).
## Transport properties
Functional characterization established nodulin 26 as a multifunctional channel rather than a simple water pore. Expression in *Xenopus* oocytes produced a mercury-sensitive increase in osmotic water permeability, and measurements on isolated soybean symbiosome membrane vesicles showed high water permeability (0.05 ± 0.003 cm/s at 20 °C), low activation energy, inhibition by 0.1 mM HgCl2, and a diffusive-to-osmotic permeability ratio of 18.3, indicating that water crosses the membrane in single-file fashion through the NOD26 channel[2](https://pubmed.ncbi.nlm.nih.gov/9195927/).
**Solute flux.** The same vesicles showed high mercury-sensitive permeability to glycerol and formamide but not urea, and the solutes that permeate NOD26 are far larger than water, so solute flux appears to occur by a pathway distinct from that of water[2](https://pubmed.ncbi.nlm.nih.gov/9195927/). When the purified protein was reconstituted into liposomes, osmotic water permeability was 0.012 ± 0.0013 cm/s, fivefold above control liposomes, and glycerol flux was 43-fold higher than in controls, with both water and glycerol transport inhibited by HgCl2 and showing low activation energies[7](https://pubmed.ncbi.nlm.nih.gov/9890916/).
**Ion conductance.** In an earlier study, nodulin 26 purified from soybean nodules and reconstituted into planar lipid bilayers formed single channels with a maximum unitary conductance of 3.1 nS in 1 M KCl, with discrete lower-conductance states from 0.5 to 2.5 nS. The channels transported both cations and anions with weak selectivity for anions and were voltage-sensitive, the high-conductance state being preferentially occupied at lower applied voltages[5](https://pubmed.ncbi.nlm.nih.gov/7517934/).
## Role in the symbiosome
In the legume-rhizobia symbiosis, rhizobia released from infection threads differentiate into nitrogen-fixing bacteroids, each enclosed by the plant-derived symbiosome membrane, a physical barrier under plant control for the movement of solutes between the partners[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC4266029/). Bacteroids convert atmospheric nitrogen to ammonia, some of which becomes ammonium in the acidic symbiosome space; ammonia translocation back across the symbiosome membrane to the plant cytosol is mediated by NOD26, which is exclusively and abundantly present in that membrane[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC8644718/). Two pathways have been proposed for fixed-nitrogen export, a monovalent cation channel for ammonium and the aquaglyceroporin NOD26 for NH3[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC6305480/).
NOD26 also serves as a docking station for cytosolic glutamine synthetase, the enzyme that assimilates ammonia; this localization would promote rapid assimilation of the exported ammonia and create a strong sink for further export[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC4266029/). Both glutamine synthetase and NOD26 were detected in the soybean symbiosome membrane proteome, supporting their proposed roles in ammonia release from the symbiosome[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC4266029/).
## The NIP family
Nodulin 26 gives its name to the nodulin 26-like intrinsic protein (NIP) family, a group of highly conserved, multifunctional major intrinsic proteins unique to plants that transport uncharged solutes ranging from water to ammonia to glycerol[3](https://www.sciencedirect.com/science/article/pii/S0005273606001118). Based on structure-function studies, the family divides into two subgroups, I and II, defined by the amino acids in the aromatic/arginine (ar/R) selectivity filter of the transport pore. Both subgroups contain multifunctional transporters with low to no water permeability, and the range of solutes transported depends on the filter residues[3](https://www.sciencedirect.com/science/article/pii/S0005273606001118).
A comparison of soybean nodulin 26 (subgroup I) with the [Arabidopsis](https://www.edgepedia.org/arabidopsis) protein AtNIP6;1 (subgroup II) showed that the two differ principally by the substitution of a conserved alanine (subgroup II) for a conserved tryptophan (subgroup I) at the helix 2 (H2) position of the ar/R filter. Nodulin 26 is an aquaglyceroporin with modest osmotic water permeability, whereas AtNIP6;1 showed no measurable water permeability but transported glycerol, formamide, and larger solutes impermeable to nodulin 26. [Site-directed mutagenesis](https://www.edgepedia.org/site-directed-mutagenesis) identified the H2 position as the crucial determinant of these behaviors; across the NIP and tonoplast intrinsic protein families, histidine at H2 marks aquaporins, tryptophan marks aquaglyceroporins, and alanine marks water-impermeable glyceroporins[10](https://pubs.acs.org/doi/10.1021/bi0511888).
NIPs tend to be expressed at low levels in the plant compared with other major intrinsic proteins, and several show cell- or tissue-specific expression regulated spatially and temporally during development[3](https://www.sciencedirect.com/science/article/pii/S0005273606001118).
## Regulation
NIPs are subject to posttranslational phosphorylation by calcium-dependent protein kinases. For soybean nodulin 26, phosphorylation stimulates transport activity and is regulated by developmental and environmental cues, including osmotic stress[3](https://www.sciencedirect.com/science/article/pii/S0005273606001118). The phosphorylation site is Ser-262, which occurs in planta and stimulates GmNOD26 water channel activity[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC8921385/).
## References
1. [Fortin, M. G., Morrison, N. A., Verma, D. P. S. (1987). Nodulin-26, a peribacteroid membrane nodulin is expressed independently of the development of the peribacteroid compartment. Nucleic Acids Research 15(2):813-824.](https://pmc.ncbi.nlm.nih.gov/articles/PMC340469/)
2. [Dean, R. M. et al. (1999). Functional analysis of nodulin 26, an aquaporin in soybean root nodule symbiosomes. Journal of Biological Chemistry 272(26):16256.](https://pubmed.ncbi.nlm.nih.gov/9195927/)
3. [Wallace, I. S., Choi, W.-G., Roberts, D. M. (2006). The structure, function and regulation of the nodulin 26-like intrinsic protein family of plant aquaglyceroporins. Biochimica et Biophysica Acta 1758(8):1165-1175.](https://www.sciencedirect.com/science/article/pii/S0005273606001118)
4. [Clarke, V. C. et al. (2015). Transport processes of the legume symbiosome membrane. Frontiers in Plant Science.](https://pmc.ncbi.nlm.nih.gov/articles/PMC4266029/)
5. [Weaver, C. D. et al. (1994). Nodulin 26, a nodule-specific symbiosome membrane protein from soybean, is an ion channel.](https://pubmed.ncbi.nlm.nih.gov/7517934/)
6. [Aquaporin ion conductance properties defined by membrane environment, protein structure, and cell physiology. PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC8921385/)
7. [Rivers, R. L. et al. (1998). Purification and functional reconstitution of soybean nodulin 26. An aquaporin with water and glycerol transport properties. Biochemistry.](https://pubmed.ncbi.nlm.nih.gov/9890916/)
8. [A roadmap of plant membrane transporters in arbuscular mycorrhizal and legume-rhizobium symbioses. PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC8644718/)
9. [Interaction and Regulation of Carbon, Nitrogen, and Phosphorus Metabolisms in Root Nodules of Legumes. PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC6305480/)
10. [Wallace, I. S., Roberts, D. M. (2005). Distinct Transport Selectivity of Two Structural Subclasses of the Nodulin-like Intrinsic Protein Family of Plant Aquaglyceroporin Channels. Biochemistry.](https://pubs.acs.org/doi/10.1021/bi0511888)
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*Topic: Encyclopedia › Life and health › Biological foundations › Cell biology › Membranes and trafficking › Membrane transport and channels › Aquaporins and water transport*
*Initially written Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license