nucleation

LIT-1560: Protein import into bacterial endosymbionts and evolving organelles

2 min read

Properties6
StatusActive — read in full 2026-10-27 (NOTE-1241) from the PubMed Central copy of the version of record (PMC12176269, open access); worth reading as the system-by-system statement of what is known about host proteins entering bacterial endosymbionts, import at scale in the Paulinella chromatophore and the nitroplast and single proteins elsewhere, for its point that gene transfer from the endosymbiont is not needed for an organelle (THEORY-1100), and for its ordered scenario of effectors, dependence and general import (THEORY-1107). Read it knowing that it is a review whose import-at-scale evidence is two proteomic studies, that most single-protein cases rest on antibody localization, that the scenario is offered as plausible on limited data, and that import mechanisms are almost entirely unknown.
Version3
Tagsendosymbiosis, natural-sciences, ecology
Cited as “paper” byNOTE-1241 — Sørensen, Stiller, Kröninger & Nowack — Protein import into bacterial endosymbionts and evolving organelles
Cited as “reported in” byCASE-167 — The nitroplast: UCYN-A, a streamlined nitrogen-fixing cyanobacterium in a marine alga, whose division is coupled to the host's and which imports host-encoded proteins, reclassified as an early-stage organelle in 2024, CASE-174 — Paulinella chromatophora: a photosynthetic organelle in the making, from an endosymbiosis independent of the plastids', its genome reduced to about a third of its coding capacity, and hundreds of host proteins imported into it, most encoded by the host's own genes rather than transferred from the symbiont
Cited as “source” byTHEORY-1100 — On Sørensen and colleagues' review, a bacterial endosymbiont becomes genetically integrated, the authors' mark of an organelle, when nuclear genes whose products are imported into it functionally replace essential genes it has lost, and this does not require gene transfer from the endosymbiont: in the Paulinella chromatophore and the nitroplast, the two systems with hundreds of imported proteins, many of which fill gaps left by lost genes, the imported proteins come mostly from the host's own genes or from other bacteria, and in the nitroplast no gene transfer from the endosymbiont has been found, THEORY-1107 — On Sørensen and colleagues' scenario, an endosymbiont becomes an organelle in steps: the host first targets effector proteins into it; an imported host protein redundant with one of its functions then lets the endosymbiont's own gene decay, making it dependent; and a general import pathway, once evolved, brings in hundreds of proteins; this genetic integration arises only in permanent, vertically transmitted associations, and host effector control without vertical transmission, as in rhizobia, does not lead to it, THEORY-1211 — An endosymbiont becomes an organelle in steps: the host first targets effector proteins into it; an imported host protein redundant with one of its functions then lets the endosymbiont's own gene decay, making it dependent; and a general import pathway, once evolved, brings in hundreds of proteins; this genetic integration arises only in permanent, vertically transmitted associations, THEORY-1228 — A bacterial endosymbiont becomes genetically integrated, an organelle, when host proteins imported into it replace genes it has lost, and this does not require gene transfer from the endosymbiont: in the Paulinella chromatophore and the nitroplast, the two systems with hundreds of imported proteins, the replacing genes are mostly the host's own or come from other bacteria

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Sørensen, M. E. S., Stiller, M. L., Kröninger, L. & Nowack, E. C. M. (2025). Protein import into bacterial endosymbionts and evolving organelles. The FEBS Journal 292(12): 2992–3013. doi:10.1111/febs.17356.

Key takeaways

From the reading, NOTE-1241.

  • Import at scale in two systems. Proteomics of isolated endosymbionts found hundreds of host proteins in Paulinella’s chromatophore and more than 350 in the nitroplast, filling gaps left by lost genes.
  • No gene transfer needed. The imported proteins come “mostly from the host itself” in Paulinella, and no EGT at all was found in the nitroplast’s host: “EGT is not a precondition for the genetic integration of an endosymbiont” (THEORY-1100).
  • Single proteins elsewhere. Six or seven targeted proteins in a diatom and a trypanosomatid, and one or two each in the aphid’s Buchnera, two weevils’ symbionts and the mealybug’s Moranella, mostly found with antibodies. The mealybug protein is in Moranella, not Tremblaya.
  • A proposed order. Effectors first, then dependence on one redundant imported protein, then a general import pathway. Rhizobia, targeted by hundreds of host peptides but not vertically transmitted, are the authors’ case of control without integration (THEORY-1107).
  • Caveats. It is a review. The scenario is called plausible on limited data, and the import mechanisms are almost entirely unknown.

Why this is here

A1 ranked it third, for the mechanisms of host–endosymbiont integration and for why protein import complicates a sharp symbiont/organelle boundary. It bears on QUESTION-277’s regulatory-integration dimension.

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