Abstract
Encapsulins are bacterial organelle-like cages involved in various aspects of metabolism, especially protection from oxidative stress. They can serve as vehicles for a wide range of medical applications. Encapsulin shell proteins are structurally similar to HK97 bacteriophage capsid protein and their function depends on the encapsulated cargos. The Myxococcus xanthus encapsulin system comprises EncA and three cargos: EncB, EncC, and EncD. EncB and EncC are similar to bacterial ferritins that can oxidize Fe+2 to less toxic Fe+3. We analyzed EncA, EncB, and EncC by cryo-EM and X-ray crystallography. Cryo-EM shows that EncA cages can have T= 3 and T= 1 symmetry and that EncA T= 1 has a unique protomer arrangement. Also, we define EncB and EncC binding sites on EncA. X-ray crystallography of EncB and EncC reveals conformational changes at the ferroxidase center and additional metal binding sites, suggesting a mechanism for Fe oxidation and storage within the encapsulin shell.
Highlights
Encapsulins are protein-based nanocompartments that have been identified in a variety of bacterial and archaeal species (Gabashvili et al, 2020; Giessen, 2016; Nichols et al, 2017)
EncA can form nanocompartments with T = 1 and T = 3 symmetry in the absence of a cargo The first structure of EncA encapsulin was obtained at 4.6 Aresolution (PDB: 4PT2, EMD-5917) using particles isolated from wild-type M. xanthus (McHugh et al, 2014)
The majority ($94%) of EncA encapsulins purified from M. xanthus were 32 nm in diameter and arranged in T = 3 symmetry (McHugh et al, 2014), whereas with E. coli expression products, we observed empty EncA shells of two sizes
Summary
Encapsulins are protein-based nanocompartments that have been identified in a variety of bacterial and archaeal species (Gabashvili et al, 2020; Giessen, 2016; Nichols et al, 2017). The smallest encapsulins have been identified in Thermotoga marritima, Mycolicibacterium hassiacum, Synechococcus elongatus, and Mycobacterium smegmatis (Loncar et al, 2020; Nichols et al, 2021; Sutter et al, 2008; Tang et al, 2021; Wiryaman and Toor, 2021). These encapsulins are formed by 60 protomers with T = 1 symmetry.
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