Cell surface proteome of the marine planctomycete Rhodopirellula baltica

An overview on enrichment methods for cell surface proteome profiling

Cell surface proteins are essential for many important biological processes, including cell-cell interactions, signal transduction, and molecular transportation. With the characteristics of low abundance, high hydrophobicity, and high heterogeneity, it is difficult to get a comprehensive view of cell surface proteome by direct analysis. Thus, it is important to selectively enrich the cell surface proteins before liquid chromatography with mass spectrometry analysis. In recent years, a variety of enrichment methods have been developed. Based on the separation mechanism, these methods could be mainly classified into three types. The first type is based on their difference in the physicochemical property, such as size, density, charge, and hydrophobicity. The second one is based on the bimolecular affinity interaction with lectin or antibody. And the third type is based on the chemical covalent coupling to free side groups of surface-exposed proteins or carbohydrate chains, such as primary amines, carboxyl groups, glycan side chains. In addition, metabolic labeling and enzymatic reaction-based methods have also been employed to selectively isolate cell surface proteins. In this review, we will provide a comprehensive overview of the enrichment methods for cell surface proteome profiling.

MALDI mass spectrometry imaging and in situ microproteomics of Listeria monocytogenes biofilms

MALDI-TOF Mass spectrometry Imaging (MSI) is a surface-sampling technology that can determine spatial information and relative abundance of analytes directly from biological samples. Human listeriosis cases are due to the ingestion of contaminated foods with the pathogenic bacteria Listeria monocytogenes. The reduction of water availability in food workshops by decreasing the air relative humidity (RH) is one strategy to improve the control of bacterial contamination. This study aims to develop and implement an MSI approach on L. monocytogenes biofilms and proof of concept using a dehumidified stress condition. MSI allowed examining the distribution of low molecular weight proteins within the biofilms subjected to a dehumidification environment, mimicking the one present in a food workshop (10 °C, 75% RH). Furthermore, a LC-MS/MS approach was made to link the dots between MSI and protein identification. Five identified proteins were assigned to registered MSI m/z, including two cold-shock proteins and a ligase involved in cell wall biogenesis. These data demonstrate how imaging can be used to dissect the proteome of an intact bacterial biofilm giving new insights into protein expression relating to a dehumidification stress adaptation. Data are available via ProteomeXchange with identifier PXD010444.

BIOLOGICAL SIGNIFICANCE

The ready-to-eat food processing industry has the daily challenge of controlling the contamination of surfaces and machines with spoilage and pathogenic microorganisms. In some cases, it is a lost cause due to these microorganisms' capacity to withstand the cleaning treatments, like desiccation procedures. Such a case is the ubiquitous Gram-positive Bacterium Listeria monocytogenes. Its surface proteins have particular importance for the interaction with its environment, being important factors contributing to adaptation to stress conditions. There are few reproducibly techniques to obtain the surface proteins of Gram-positive cells. Here, we developed a workflow that enables the use of MALDI imaging on Gram-positive bacterium biofilms to study the impact of dehumidification on sessile cells. It will be of the most interest to test this workflow with different environmental conditions and potentially apply it to other biofilm-forming bacteria.

Evolutionary gradient of predicted nuclear localization signals (NLS)-bearing proteins in genomes of family Planctomycetaceae

BACKGROUND

The nuclear envelope is considered a key classification marker that distinguishes prokaryotes from eukaryotes. However, this marker does not apply to the family Planctomycetaceae, which has intracellular spaces divided by lipidic intracytoplasmic membranes (ICMs). Nuclear localization signal (NLS), a short stretch of amino acid sequence, destines to transport proteins from cytoplasm into nucleus, and is also associated with the development of nuclear envelope. We attempted to investigate the NLS motifs in Planctomycetaceae genomes to demonstrate the potential molecular transition in the development of intracellular membrane system.

RESULTS

In this study, we identified NLS-like motifs that have the same amino acid compositions as experimentally identified NLSs in genomes of 11 representative species of family Planctomycetaceae. A total of 15 NLS types and 170 NLS-bearing proteins were detected in the 11 strains. To determine the molecular transformation, we compared NLS-bearing protein abundances in the 11 representative Planctomycetaceae genomes with them in genomes of 16 taxonomically varied microorganisms: nine bacteria, two archaea and five fungi. In the 27 strains, 29 NLS types and 1101 NLS-bearing proteins were identified, principal component analysis showed a significant transitional gradient from bacteria to Planctomycetaceae to fungi on their NLS-bearing protein abundance profiles. Then, we clustered the 993 non-redundant NLS-bearing proteins into 181 families and annotated their involved metabolic pathways. Afterwards, we aligned the ten types of NLS motifs from the 13 families containing NLS-bearing proteins among bacteria, Planctomycetaceae or fungi, considering their diversity, length and origin. A transition towards increased complexity from non-planctomycete bacteria to Planctomycetaceae to archaea and fungi was detected based on the complexity of the 10 types of NLS-like motifs in the 13 NLS-bearing proteins families.

CONCLUSION

The results of this study reveal that Planctomycetaceae separates slightly from the members of non-planctomycete bacteria but still has substantial differences from fungi, based on the NLS-like motifs and NLS-bearing protein analysis.

Nuclear Pore-Like Structures in a Compartmentalized Bacterium

Planctomycetes are distinguished from other Bacteria by compartmentalization of cells via internal membranes, interpretation of which has been subject to recent debate regarding potential relations to Gram-negative cell structure. In our interpretation of the available data, the planctomycete Gemmata obscuriglobus contains a nuclear body compartment, and thus possesses a type of cell organization with parallels to the eukaryote nucleus. Here we show that pore-like structures occur in internal membranes of G.obscuriglobus and that they have elements structurally similar to eukaryote nuclear pores, including a basket, ring-spoke structure, and eight-fold rotational symmetry. Bioinformatic analysis of proteomic data reveals that some of the G. obscuriglobus proteins associated with pore-containing membranes possess structural domains found in eukaryote nuclear pore complexes. Moreover, immunogold labelling demonstrates localization of one such protein, containing a β-propeller domain, specifically to the G. obscuriglobus pore-like structures. Finding bacterial pores within internal cell membranes and with structural similarities to eukaryote nuclear pore complexes raises the dual possibilities of either hitherto undetected homology or stunning evolutionary convergence.

Outer Membrane Proteins Derived from Non-cyanobacterial Lineage Cover the Peptidoglycan of Cyanophora paradoxa Cyanelles and Serve as a Cyanelle Diffusion Channel

The cyanelle is a primitive chloroplast that contains a peptidoglycan layer between its inner and outer membranes. Despite the fact that the envelope structure of the cyanelle is reminiscent of Gram-negative bacteria, the Cyanophora paradoxa genome appears to lack genes encoding homologs of putative peptidoglycan-associated outer membrane proteins and outer membrane channels. These are key components of Gram-negative bacterial membranes, maintaining structural stability and regulating permeability of outer membrane, respectively. Here, we discovered and characterized two dominant peptidoglycan-associated outer membrane proteins of the cyanelle (∼2 × 10(6) molecules per cyanelle). We named these proteins CppF and CppS (cyanelle peptidoglycan-associated proteins). They are homologous to each other and function as a diffusion channel that allows the permeation of compounds with Mr <1,000 as revealed by permeability measurements using proteoliposomes reconstituted with purified CppS and CppF. Unexpectedly, amino acid sequence analysis revealed no evolutionary linkage to cyanobacteria, showing only a moderate similarity to cell surface proteins of bacteria belonging to Planctomycetes phylum. Our findings suggest that the C. paradoxa cyanelle adopted non-cyanobacterial lineage proteins as its main outer membrane components, providing a physical link with the underlying peptidoglycan layer and functioning as a diffusion route for various small substances across the outer membrane.

Sample preparation strategies for improving the identification of membrane proteins by mass spectrometry

Despite enormous advances in the mass spectrometry and proteomics fields during the last two decades, the analysis of membrane proteins still remains a challenge for the proteomic community. Membrane proteins play a wide number of key roles in several cellular events, making them relevant target molecules to study in a significant variety of investigations (e.g., cellular signaling, immune surveillance, drug targets, vaccine candidates, etc.). Here, we critically review the several attempts that have been carried out on the different steps of the sample preparation procedure to improve and modify existing conventional proteomic strategies in order to make them suitable for the study of membrane proteins. We also revise novel techniques that have been designed to tackle the difficult but relevant task of identifying and characterizing membrane proteins.

Bioinformatic analyses of integral membrane transport proteins encoded within the genome of the planctomycetes species, Rhodopirellula baltica

Rhodopirellula baltica (R. baltica) is a Planctomycete, known to have intracellular membranes. Because of its unusual cell structure and ecological significance, we have conducted comprehensive analyses of its transmembrane transport proteins. The complete proteome of R. baltica was screened against the Transporter Classification Database (TCDB) to identify recognizable integral membrane transport proteins. 342 proteins were identified with a high degree of confidence, and these fell into several different classes. R. baltica encodes in its genome channels (12%), secondary carriers (33%), and primary active transport proteins (41%) in addition to classes represented in smaller numbers. Relative to most non-marine bacteria, R. baltica possesses a larger number of sodium-dependent symporters but fewer proton-dependent symporters, and it has dimethylsulfoxide (DMSO) and trimethyl-amine-oxide (TMAO) reductases, consistent with its Na(+)-rich marine environment. R. baltica also possesses a Na(+)-translocating NADH:quinone dehydrogenase (Na(+)-NDH), a Na(+) efflux decarboxylase, two Na(+)-exporting ABC pumps, two Na(+)-translocating F-type ATPases, two Na(+):H(+) antiporters and two K(+):H(+) antiporters. Flagellar motility probably depends on the sodium electrochemical gradient. Surprisingly, R. baltica also has a complete set of H(+)-translocating electron transport complexes similar to those present in α-proteobacteria and eukaryotic mitochondria. The transport proteins identified proved to be typical of the bacterial domain with little or no indication of the presence of eukaryotic-type transporters. However, novel functionally uncharacterized multispanning membrane proteins were identified, some of which are found only in Rhodopirellula species, but others of which are widely distributed in bacteria. The analyses lead to predictions regarding the physiology, ecology and evolution of R. baltica.

Proteinaceous determinants of surface colonization in bacteria: bacterial adhesion and biofilm formation from a protein secretion perspective

Bacterial colonization of biotic or abiotic surfaces results from two quite distinct physiological processes, namely bacterial adhesion and biofilm formation. Broadly speaking, a biofilm is defined as the sessile development of microbial cells. Biofilm formation arises following bacterial adhesion but not all single bacterial cells adhering reversibly or irreversibly engage inexorably into a sessile mode of growth. Among molecular determinants promoting bacterial colonization, surface proteins are the most functionally diverse active components. To be present on the bacterial cell surface, though, a protein must be secreted in the first place. Considering the close association of secreted proteins with their cognate secretion systems, the secretome (which refers both to the secretion systems and their protein substrates) is a key concept to apprehend the protein secretion and related physiological functions. The protein secretion systems are here considered in light of the differences in the cell-envelope architecture between diderm-LPS (archetypal Gram-negative), monoderm (archetypal Gram-positive) and diderm-mycolate (archetypal acid-fast) bacteria. Besides, their cognate secreted proteins engaged in the bacterial colonization process are regarded from single protein to supramolecular protein structure as well as the non-classical protein secretion. This state-of-the-art on the complement of the secretome (the secretion systems and their cognate effectors) involved in the surface colonization process in diderm-LPS and monoderm bacteria paves the way for future research directions in the field.

Exoproteomics: exploring the world around biological systems

The term 'exoproteome' describes the protein content that can be found in the extracellular proximity of a given biological system. These proteins arise from cellular secretion, other protein export mechanisms or cell lysis, but only the most stable proteins in this environment will remain in abundance. It has been shown that these proteins reflect the physiological state of the cells in a given condition and are indicators of how living systems interact with their environments. High-throughput proteomic approaches based on a shotgun strategy, and high-resolution mass spectrometers, have modified the authors' view of exoproteomes. In the present review, the authors describe how these new approaches should be exploited to obtain the maximum useful information from a sample, whatever its origin. The methodologies used for studying secretion from model cell lines derived from eukaryotic, multicellular organisms, virulence determinants of pathogens and environmental bacteria and their relationships with their habitats are illustrated with several examples. The implication of such data, in terms of proteogenomics and the discovery of novel protein functions, is discussed.

Surfomics: shaving live organisms for a fast proteomic identification of surface proteins

Surface proteins play a critical role in the interaction between cells and their environment, as they take part in processes like signaling, adhesion, transport, etc. In pathogenic microorganisms, they can also participate in virulence or cytotoxicity. As these proteins have the highest chances to be recognized by the immune system, they are often the targets for the discovery of new vaccines. In addition, they can serve for the development of serological-based tools to diagnose infectious diseases. First-generation proteomic strategies for the identification of surface proteins rely on the biochemical fractionation and/or enrichment of this group of molecules or organelles containing them. However, in the last years, a novel second-generation approach has been developed, consisting of the digestion of live, intact cells with proteases, so that surface-exposed moieties (i.e. the "surfome" of a cell) are "shaved" and analyzed by LC/MS/MS. Here we review such a strategy, firstly set up and developed in Gram-positive bacteria, and further applied to Gram-negative bacteria, unicellular fungi, and also pluricellular organisms. We also discuss the advantages and inconvenients of the approach, and the still unresolved question about the intriguing presence of proteins predicted as cytoplasmic in the surfomes. This article is part of a Special Issue entitled: Trends in Microbial Proteomics.