# Exploring protein translocation by the Legionella pneumophila Dot/Icm Type IV Section System

> **NIH NIH R21** · UNIVERSITY OF MICHIGAN AT ANN ARBOR · 2021 · $234,000

## Abstract

Project Summary
 Bacterial pathogens represent an increasing threat to global health due to the growing problem of
pathogen drug resistance. Although the mechanisms deployed by pathogens to infect hosts are diverse, a
common obstacle that pathogenic bacteria must overcome is moving virulence factors across multiple
membrane barriers – both their own and the host cell’s. This process represents a potential bacterial “Achilles
heel” for inhibiting pathogenesis. One potent bacterial weapon that accomplishes this feat is the Type IV
Secretion System (T4SS), a large complex, composed of 12-30 components depending on the bacteria, that
spans the bacterial inner and outer membranes. In Gram-negative bacteria these complexes can deliver
effector proteins into eukaryotic cells, DNA into other bacteria, and/or toxins into bacterial neighbors. We
purified and determined the first high-resolution structure of the Legionella pneumophila Dot/Icm (defect in
organelle transport/ intracellular multiplication) T4SS using single particle cryo-electron microscopy (cryo-EM)
allowing us to build atomic models of T4SS components. L. pneumophila is an opportunistic pathogen that
infects lung macrophages leading to a potentially fatal pneumonia called Legionnaires’ Disease and the
Dot/Icm T4SS is required for pathogenesis. Discoveries from our work on the Dot/Icm T4SS include the
identification of a previously unrecognized core T4SS component, identification and characterization of
symmetry mismatches between the outer membrane cap (OMC) and periplasmic ring (PR), and an unexpected
molar organization of components in the OMC. Despite this progress, many questions remain. The resolution
of our Dot/Icm T4SS structure was not high enough to build a complete model of all the regions in our density
map, our purification clearly lacks major structural components seen in in situ cryo-electron tomography
studies of intact L. pneumophila, and there is currently no molecular understanding for how the T4SS from any
organism identifies, engages, and moves proteins across membranes. The purpose of this proposal is to
understand on a molecular level how the Dot/Icm T4SS translocates proteins. To reach this goal we need a
more detailed map of the Dot/Icm T4SS, new ways to purify the complex that preserve additional structural
features, and the ability to begin structurally and biochemically interrogating substrate translocation by the
T4SS. While reaching these benchmarks will require the successful completion of high-risk and challenging
experiments, progress made on any of these goals will provide impactful information about the molecular
organization of this complex T4SS, results required for understanding how T4SSs are tuned to translocate
specific substrates.

## Key facts

- **NIH application ID:** 10314686
- **Project number:** 1R21AI164651-01
- **Recipient organization:** UNIVERSITY OF MICHIGAN AT ANN ARBOR
- **Principal Investigator:** Melanie Diane Ohi
- **Activity code:** R21 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2021
- **Award amount:** $234,000
- **Award type:** 1
- **Project period:** 2021-06-11 → 2023-05-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10314686

## Citation

> US National Institutes of Health, RePORTER application 10314686, Exploring protein translocation by the Legionella pneumophila Dot/Icm Type IV Section System (1R21AI164651-01). Retrieved via AI Analytics 2026-05-22 from https://api.ai-analytics.org/grant/nih/10314686. Licensed CC0.

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