Predicted structure of the 89-kDa invasion-tip protein of Holospora obtusa, an endonuclear symbiont of Paramecium caudatum, suggests a pH-dependent conformational change
- 1Yamaguchi University
Abstract
The endonuclear symbiont Holospora obtusa invades the macronucleus of Paramecium caudatum using an infectious-form invasion-tip containing a major 89-kDa periplasmic protein implicated in escape from the host digestive vacuole and macronuclear invasion. Sequence-based analyses placed this protein within a Holospora-specific family and predicted an N-terminal membrane-anchoring module and a long alpha-helical C-terminal region. AlphaFold2 modeling followed by molecular dynamics indicated that the C-terminal region forms an antiparallel, spectrin-repeat-like helical solenoid containing a localized three-helix coiled-coil. The protein exhibits a large pH-dependent charge difference, becoming 55 units more positive at pH 4 than at pH 7. Independent simulations at these pH values differed mainly in the relative orientation of the N-terminal core and C-terminal rod, described by DynDom as an approximately 35 degrees rigid-body rotation about a hinge centered on Gly291. However, none of the nearby glutamates (Glu273, Glu275, and Glu302) formed a pH-dependent salt bridge, and no specific protonation-dependent interaction responsible for the conformational difference was identified. We nonetheless propose, as a testable hypothesis, that acidification of the digestive vacuole may promote this reorientation, thereby repositioning the N-terminal membrane-anchoring module at the bacterial surface as an early step toward escape from the vacuole.
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BibTeX
@misc{takenaka2026predicted,
title = {{Predicted structure of the 89-kDa invasion-tip protein of Holospora obtusa, an endonuclear symbiont of Paramecium caudatum, suggests a pH-dependent conformational change}},
author = {Y. Takenaka and T. Murai and T. Nakatsu and Y. Shigeri and M. Fujishima},
howpublished = {bioRxiv},
year = {2026},
month = oct,
doi = {10.64898/2026.10.08.757700},
url = {https://www.biorxiv.org/content/10.64898/2026.10.08.757700v1},
note = {CC BY-NC-ND},
}
RIS
TY - UNPB
TI - Predicted structure of the 89-kDa invasion-tip protein of Holospora obtusa, an endonuclear symbiont of Paramecium caudatum, suggests a pH-dependent conformational change
AU - Y. Takenaka
AU - T. Murai
AU - T. Nakatsu
AU - Y. Shigeri
AU - M. Fujishima
PY - 2026
DA - 2026/10/08
PB - bioRxiv
DO - 10.64898/2026.10.08.757700
UR - https://www.biorxiv.org/content/10.64898/2026.10.08.757700v1
AB - The endonuclear symbiont Holospora obtusa invades the macronucleus of Paramecium caudatum using an infectious-form invasion-tip containing a major 89-kDa periplasmic protein implicated in escape from the host digestive vacuole and macronuclear invasion. Sequence-based analyses placed this protein within a Holospora-specific family and predicted an N-terminal membrane-anchoring module and a long alpha-helical C-terminal region. AlphaFold2 modeling followed by molecular dynamics indicated that the C-terminal region forms an antiparallel, spectrin-repeat-like helical solenoid containing a localized three-helix coiled-coil. The protein exhibits a large pH-dependent charge difference, becoming 55 units more positive at pH 4 than at pH 7. Independent simulations at these pH values differed mainly in the relative orientation of the N-terminal core and C-terminal rod, described by DynDom as an approximately 35 degrees rigid-body rotation about a hinge centered on Gly291. However, none of the nearby glutamates (Glu273, Glu275, and Glu302) formed a pH-dependent salt bridge, and no specific protonation-dependent interaction responsible for the conformational difference was identified. We nonetheless propose, as a testable hypothesis, that acidification of the digestive vacuole may promote this reorientation, thereby repositioning the N-terminal membrane-anchoring module at the bacterial surface as an early step toward escape from the vacuole.
ER -