A quantitative assessment of the membrane-integral sub-proteome of a bacterial magnetic organelle.

TLDR: The results suggest that the magnetosome membrane is specifically associated with a small subset of integral proteins that are tightly packed within the lipid layer that will help to elucidate the complex process of magnetosomes biogenesis.

About: This article is published in Journal of Proteomics.The article was published on 2018-02-10 and is currently open access. It has received 34 citations till now. The article focuses on the topics: Magnetosome & Organelle assembly.

Figures

Figure 1: Fractionation workflow of magnetically separated magnetosomes, 860 stringently washed magnetosomes, non-magnetic cell lysate and enriched non-861 magnetic membrane fraction. Additionally, workflow to obtain transmembrane 862 peptides from magnetosome proteins by Proteinase K membrane shaving is outlined. 863 UC: Ultracentrifugation 864 865

Figure 2: Cellular localization of six Mam/Mms proteins in M. gryphiswaldense. All 867 proteins were C- or N-terminally fused to EGFP and expressed in the wild type. A 30 868 amino acid alpha-helical linker (HL) was placed in between the fusion proteins. A) 869 C-terminal PmamDC-mms6-HL-egfp construct, expressed from plasmid. B) N-terminal 870 PmamDC-mmsF-HL-egfp construct, expressed from plasmid. C) N-terminal PmamDC-871 mamE-HL-egfp construct, expressed from ectopic chromosomal locus. D) C-terminal 872 PmamDC-mamR-HL-egfp construct, expressed from plasmid. N-terminal fusion 873 exhibited comparable localization pattern. E) C-terminal mamA-HL-egfp construct, 874 expressed from native chromosomal locus (in-frame gene fusion) F) C-terminal 875 PmamDC-mamW-HL-egfp construct, expressed from plasmid. Fluorescence (left) and 876 corresponding differential interference contrast (right) images are shown. Scale bar: 877 2µm 878 879

Table 2: Top20-abundant and all assigned genuine magnetosome proteins. The 904 following parameters were applied: Amem/Anon-mag ≥ 1, Amag/Amem > 4, Amag.str/Amag ≥ 905 1. Abundance relative to MamC was calculated by Amag.str/Amag.str(MamC) value. The 906 number of predicted protein copies is estimated by assuming a MamC copy number 907 of 100 per magnetosome [21]. Genuine magnetosome-membrane proteins meeting 908 the parameter criteria are marked by grey columns; other proteins only show high 909 abundance values compared to MamC copy number. 910

Table 4: Coverage of the magnetosome membrane surface by transmembrane 918 domains of integral magnetosome proteins 919

Figure 3: Model of transmembrane helices (domains) embedded in a membrane. 881 Transmembrane helices (domains) are colored in red, primary boundary lipids in 882 blue, secondary boundary lipids in green and non-interacting (‘free’) lipids in grey. 883 Approximate cross section diameters of several entities are indicated within the 884 figure. A) Top view of single transmembrane helix with two layers of boundary 885 lipids. B) Side view of transmembrane helix with two layers of boundary lipids and 886 one layer of free lipids. C) Side view of three transmembrane helices of a single 887 protein all embedded by boundary lipids. D) Side view of three transmembrane 888 helices of a single protein packed in one domain embedded by boundary lipids but 889 without internal lipids. E) Top view of an array of transmembrane domains of several 890 proteins surrounded by variable numbers of boundary lipids. One of these proteins is 891 a packed protein with transmembrane domains consisting of three transmembrane 892 helices (tetra-span) and another protein with seven (hepta-span) helices. 893 894

Table 3: Additional candidate genuine magnetosome membrane proteins 913 predicted by less stringent filter parameters. Proteins here are only listed if not 914 already mentioned in Table 2. 915

Frequently asked questions

Q1. What have the authors contributed in "A quantitative assessment of the membrane-integral sub-proteome" ?

25 Magnetotactic bacteria produce chains of complex membrane-bound organelles that 26 direct the biomineralization of magnetic nanoparticles and serve for magnetic field 27 navigation. These magnetosome compartments have recently emerged as a model for 28 studying the subcellular organization of prokaryotic organelles. In order to quantify and unambiguously identify all proteins specifically targeted to 33 the magnetosome membrane of the Alphaproteobacterium Magnetospirillum 34 gryphiswaldense, the authors analyzed the protein composition of several cellular fractions 35 by semi-quantitative mass spectrometry. This high abundance suggests 41 an unusually crowded protein composition of the membrane and a tight packing with 42 transmembrane domains of integral proteins. Their findings will help to further define 43 the structure of the organelle and contribute to the elucidation of magnetosome 44 

Q2. how many ms proteins are covered by the magnetosome?

Table S 6: Coverage of the magnetosome membrane surface by 940 transmembrane domains of integral magnetosome proteins calculated according to 941 different quantification methods 942suppl.