{
  "id": 13384808,
  "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",
  "url": "https://urgent.news/2026/10/10/predicted-structure-of-the-89-kda-invasion-tip-protein-of-holospora",
  "topic": "science",
  "section": "Science",
  "published": "2026-10-10T00:00:00.000Z",
  "source": {
    "name": "bioRxiv",
    "slug": "biorxiv",
    "url": "https://www.biorxiv.org/content/10.64898/2026.10.08.757700v1?rss=1"
  },
  "original_language": "en",
  "account": "The endonuclear symbiont Holospora obtusa employs an invasion-tip protein, weighing 89-kDa, to penetrate the macronucleus of Paramecium caudatum. This protein, part of a unique Holospora family, aids in the escape from the host's digestive vacuole and invasion into the macronucleus. Sequence-based research positioned this protein within the Holospora-specific family and forecasted an N-terminal membrane-anchoring domain and a lengthy alpha-helical C-terminal segment. AlphaFold2 modeling, complemented by molecular dynamics simulations, revealed that the C-terminal region forms a spiral structure akin to spectrin, featuring a localized trio of coiled-coil helices. The protein demonstrates a substantial pH-dependent charge variation, becoming 55 units more positively charged at pH 4 compared to pH 7. Differences in simulation outcomes at these pH levels primarily stem from the relative positioning of the N-terminal core and C-terminal rod, characterized by a near 35-degree rotation about a hinge centered on Gly291. However, none of the proximate glutamate residues (Glu273, Glu275, and Glu302) form a pH-dependent salt bridge, and no specific protonation-dependent interaction that underlies the conformational shift has been identified. Despite this, the authors propose, as a testable hypothesis, that acidification of the digestive vacuole could facilitate this reorientation, effectively relocating the N-terminal membrane-anchoring module to the bacterial surface—an initial step towards vacuole escape.",
  "summary": "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…",
  "key_points": [],
  "editors_take": null,
  "illustration": null,
  "coverage": {
    "outlets": 1,
    "also_reported_by": []
  },
  "ai_generated": true,
  "disclaimer": "Summaries, key points and the editor’s take are written by software from other outlets’ reporting and may contain errors — always check the linked original."
}