8TM8

Monomer structure of monellin loop1 mutant (YENKG)

Summary for 8TM8

• Entry DOI: 10.2210/pdb8tm8/pdb
• Related: 7D75
• Descriptor: Monellin chain B,Monellin chain A, SODIUM ION (3 entities in total)
• Functional Keywords: single-chain monellin, sweet protein, domain swapping, plant protein
• Biological source: Dioscoreophyllum cumminsii (serendipity berry); More
• Total number of polymer chains: 2
• Total formula weight: 21418.47

Authors

Manjula, R.,Pavithra, G.C.,Ramaswamy, S.,Gosavi, S. (deposition date: 2023-07-28, release date: 2024-12-18, Last modification date: 2026-07-22)

Primary citation

Manjula, R.,Chockalingam, N.,Subramanian, R.,Gosavi, S.
On proline isomerization and 3D-domain-swapping.
Biochem.Biophys.Res.Commun., 821:153872-153872, 2026

PubMed Abstract: 3D-domain-swapping is the exchange of identical "domains" between two protein monomers, which leads to homodimerization. This domain exchange can be facilitated by a single hinge-loop extending out. Hinge-loop prolines have been associated with domain-swapping, but their role remains unclear. Previously, we had engineered three domain-swapping variants of the monomeric monellin by replacing the wild-type hinge-loop sequence (L1:YENEGFREIKG) with QEVKG, YEIKG or QVVAG. The central residues of these sequences are hydrophobic (V/I), and lie at the apex of a tight solvent-exposed hinge-loop (L1Δ6) connecting two β-strands (β2-β3). The hydrophobic residue-solvent interaction likely impedes the closure of L1Δ6 and the formation of intra-chain β2-β3 contacts. This promotes domain-swapping. Here, we replaced the apex residue with the borderline-hydrophobic proline and found that it reduced domain-swapping in all three variants, with significant domain-swapping observed only in the most hydrophobic QVPAG construct. We then structurally characterized three monomeric (QEPKG, YEPKG, and QVPAG) and one domain-swapped dimeric (QVPAG) variants of monellin using X-ray crystallography. Interestingly, we find that the dimer has a trans-proline isomer, whereas all three monomers have a cis-proline. Thus, introducing proline into a solvent-exposed tight β-turn may be a robust method for designing cis-proline. Conversely, mutating such a naturally-occurring cis-proline to a hydrophobic amino acid may induce domain-swapping. The trans-proline in a hydrophobic hinge-loop (e.g. QVPAG), can provide rigidity to the domain-swapped dimer enabling the precise design of domain-swapping-driven protein assemblies. Overall, our mutational-design strategy is a step towards clarifying the role of prolines in 3D-domain-swapping and the rational design of proline isomerization.

PubMed: 42102672
DOI: 10.1016/j.bbrc.2026.153872

Experimental method

X-RAY DIFFRACTION (2.26 Å)