Proteomic studies in plants

Proteomics is a leading technology for the high-throughput analysis of proteins on a genome-wide scale. With the completion of genome sequencing projects and the development of analytical methods for protein characterization, proteomics has become a major field of functional genomics. The initial objective of proteomics was the large-scale identification of all protein species in a cell or tissue. The applications are currently being extended to analyze various functional aspects of proteins such as post-translational modifications, protein-protein interactions, activities and structures. Whereas the proteomics research is quite advanced in animals and yeast as well as Escherichia coli, plant proteomics is only at the initial phase. Major studies of plant proteomics have been reported on subcellular proteomes and protein complexes (e.g. proteins in the plasma membranes, chloroplasts, mitochondria and nuclei). Here several plant proteomics studies will be presented, followed by a recent work using multidimensional protein identification technology (MudPIT).     

Zooming in: fractionation strategies in proteomics

The recent development of mass spectrometry, i.e., high sensitivity, automation of protein identification and some post-translational modifications (PTMs) significantly increased the number of large-scale proteomics projects. However, there are still considerable limitations as none of the currently available proteomics techniques allows the analysis of an entire proteome in a single step procedure. On the other hand, there are several successful studies analyzing well defined groups of proteins, e.g., proteins of purified organelles, membrane microdomains or isolated proteins with certain PTMs. Coupling of advanced separation methodologies (different prefractionation strategies, such as subcellular fractionation, affinity purification, fractionation of proteins and peptides according to their physicochemical properties) to highly sensitive mass spectrometers provides powerful means to detect and analyze dynamic changes of low abundant regulatory proteins in eukaryotic cells on the subcellular level. This review summarizes and discusses recent strategies in proteomics approaches where different fractionation strategies were successfully applied.     

Immunoisolation and subfractionation of synaptic vesicle proteins

Within recent years, the advances in proteomics techniques have resulted in considerable novel insights into the protein expression patterns of specific tissues, cells, and organelles. The information acquired from large-scale proteomics approaches indicated, however, that the proteomic analysis of whole cells or tissues is often not suited to fully unravel the proteomes of individual organellar constituents or to identify proteins that are present at low copy numbers. In addition, the identification of hydrophobic proteins is still a challenge. Therefore, the development of techniques applicable for the enrichment of low-abundance membrane proteins is essential for a comprehensive proteomic analysis. In addition to the enrichment of particular subcellular structures by subcellular fractionation, the spectrum of techniques applicable for proteomics research can be extended toward the separation of integral and peripheral membrane proteins using organic solvents, detergents, and detergent-based aqueous two-phase systems with water-soluble polymers. Here, we discuss the efficacy of a number of experimental protocols. We demonstrate that the appropriate selection of physicochemical conditions results in the isolation of synaptic vesicles of high purity whose proteome can be subfractionated into integral membrane proteins and soluble proteins by several phase separation techniques.     

Differential proteomics of plant development

In this mini-review, recent advances in plant developmental proteomics are summarized. The growing interest in plant proteomics continually produces large numbers of developmental studies on plant cell division, elongation, differentiation, and formation of various organs. The brief overview of changes in proteome profiles emphasizes the participation of stress-related proteins in all developmental processes, which substantially changes the view on functional classification of these proteins. Next, it is noteworthy that proteomics helped to recognize some metabolic and housekeeping proteins as important signaling inducers of developmental pathways. Further, cell division and elongation are dependent on proteins involved in membrane trafficking and cytoskeleton dynamics. These protein groups are less prevalently represented in studies concerning cell differentiation and organ formation, which do not target primarily cell division. The synthesis of new proteins, generally observed during developmental processes, is followed by active protein folding. In this respect, disulfide isomerase was found to be commonly up-regulated during several developmental processes. The future progress in plant proteomics requires new and/or complementary approaches including cell fractionation, specific chemical treatments, molecular cloning and subcellular localization of proteins combined with more sensitive methods for protein detection and identification.     

Interaction Proteomics

The term proteome is traditionally associated with the identification of a large number of proteins within complex mixtures originating from a given organelle, cell or even organism. Current proteome investigations are basically focused on two major areas, expression proteomics and functional proteomics. Both approaches rely on the fractionation of protein mixtures essentially by two-dimensional polyacrylamide gel electrophoresis (2D-gel) and the identification of individual protein bands by mass spectrometric techniques (2D-MS). Functional proteomics approaches are basically addressing two main targets, the elucidation of the biological function of unknown proteins and the definition of cellular mechanisms at the molecular level. In the cell many processes are governed not only by the relative abundance of proteins but also by rapid and transient regulation of activity, association and localization of proteins and protein complexes. The association of an unknown protein with partners belonging to a specific protein complex involved in a particular process would then be strongly suggestive of its biological function. The identification of interacting proteins in stable complexes in a cellular system is essentially achieved by affinity-based procedures. Different strategies relying on this simple concept have been developed and a brief overview of the main approaches presently used in functional proteomics studies is described.     

An alternative strategy to determine the mitochondrial proteome using sucrose gradient fractionation and 1D PAGE on highly purified human heart mitochondria

An alternative strategy for mitochondrial proteomics is described that is complementary to previous investigations using 2D PAGE techniques. The strategy involves (a) obtaining highly purified preparations of human heart mitochondria using metrizamide gradients to remove cytosolic and other subcellular contaminant proteins; (b) separation of mitochondrial protein complexes using sucrose density gradients after solubilization with n-dodecyl-beta-D-maltoside; (c) 1D electrophoresis of the sucrose gradient fractions; (d) high-throughput proteomics using robotic gel band excision, in-gel digestion, MALDI target spotting and automated spectral acquisition; and (e) protein identification from mixtures of tryptic peptides by high-precision peptide mass fingerprinting. Using this approach, we rapidly identified 82 bona fide or potential mitochondrial proteins, 40 of which have not been previously reported using 2D PAGE techniques. These proteins include small complex I and complex IV subunits, as well as very basic and hydrophobic transmembrane proteins such as the adenine nucleotide translocase that are not recovered in 2D gels. The technique described here should also be useful for the identification of new protein-protein associations as exemplified by the validation of a recently discovered complex that involves proteins belonging to the prohibitin family.     

Proteomics of organelles and large cellular structures

The mass-spectrometry-based identification of proteins has created opportunities for the study of organelles, transport intermediates and large subcellular structures. Traditional cell-biology techniques are used to enrich these structures for proteomics analyses, and such analyses provide insights into the biology and functions of these structures. Here, we review the state-of-the-art proteomics techniques for the analysis of subcellular structures and discuss the biological insights that have been derived from such studies.     

Proteomics of the lysosome

Defects in lysosomal function have been associated with numerous monogenic human diseases typically classified as lysosomal storage diseases. However, there is increasing evidence that lysosomal proteins are also involved in more widespread human diseases including cancer and Alzheimer disease. Thus, there is a continuing interest in understanding the cellular functions of the lysosome and an emerging approach to this is the identification of its constituent proteins by proteomic analyses. To date, the mammalian lysosome has been shown to contain approximately 60 soluble luminal proteins and approximately 25 transmembrane proteins. However, recent proteomic studies based upon affinity purification of soluble components or subcellular fractionation to obtain both soluble and membrane components suggest that there may be many more of both classes of protein resident within this organelle than previously appreciated. Discovery of such proteins has important implications for understanding the function and the dynamics of the lysosome but can also lead the way towards the discovery of the genetic basis for human diseases of hitherto unknown etiology. Here, we describe current approaches to lysosomal proteomics and data interpretation and review the new lysosomal proteins that have recently emerged from such studies.     

利用串联质谱鉴定氨基酸突变的生物信息学算法

氨基酸突变能够改变蛋白的结构和功能,影响生物体的生命过程.基于串联质谱的鸟枪法蛋白质组学是目前大规模研究蛋白质组学的主要方法,但是现有的质谱数据鉴定流程为了提高鉴定结果的灵敏度往往会有意压缩数据库中的氨基酸突变信息.因此,如何挖掘数据中的氨基酸突变信息成为当前质谱数据鉴定的一个重要部分.当前应用于氨基酸突变鉴定的串联质谱鉴定方法大致可以分为3大类:基于序列数据库搜索的方法、基于序列标签搜索的算法以及基于图谱库搜索的算法.本文首先详细介绍了这3种氨基酸突变鉴定算法,并分析了各种方法的特点和不足,然后介绍了氨基酸突变鉴定的研究现状和发展方向.随着基于串联质谱的蛋白质组学的不断发展,蛋白序列中的氨基酸突变信息将被更好地解析出来,从而得以深入探讨由氨基酸突变引起的蛋白结构和功能改变,为揭示氨基酸突变的生物学意义奠定基础.     

Reanalysis of ProteomicsDB Using an Accurate,Sensitive, and Scalable False Discovery Rate Estimation Approach for Protein Groups

Estimating false discovery rates (FDRs) of protein identification continues to be an important topic in mass spectrometry–based proteomics, particularly when analyzing very large datasets. One performant method for this purpose is the Picked Protein FDR approach which is based on a target-decoy competition strategy on the protein level that ensures that FDRs scale to large datasets. Here, we present an extension to this method that can also deal with protein groups, that is, proteins that share common peptides such as protein isoforms of the same gene. To obtain well-calibrated FDR estimates that preserve protein identification sensitivity, we introduce two novel ideas. First, the picked group target-decoy and second, the rescued subset grouping strategies. Using entrapment searches and simulated data for validation, we demonstrate that the new Picked Protein Group FDR method produces accurate protein group-level FDR estimates regardless of the size of the data set. The validation analysis also uncovered that applying the commonly used Occam’s razor principle leads to anticonservative FDR estimates for large datasets. This is not the case for the Picked Protein Group FDR method. Reanalysis of deep proteomes of 29 human tissues showed that the new method identified up to 4% more protein groups than MaxQuant. Applying the method to the reanalysis of the entire human section of ProteomicsDB led to the identification of 18,000 protein groups at 1% protein group-level FDR. The analysis also showed that about 1250 genes were represented by ≥2 identified protein groups. To make the method accessible to the proteomics community, we provide a software tool including a graphical user interface that enables merging results from multiple MaxQuant searches into a single list of identified and quantified protein groups.     

Proteomics analysis of starved cells revealed Annexin A1 as an important regulator of autophagic degradation

Macroautophagy is involved in the bulk degradation of long-lived cytosolic proteins and subcellular organelles, which is important for the survival of cells during starvation. To identify potential players of the autophagy process, we subjected HCT116 cells cultured in complete medium and in Earle’s balanced salt solution to proteomics analysis. In approximately 1500 protein spots detected, we characterized 52 unique proteins, whose expression levels were significantly changed following starvation. Notably, we found that Annexin A1 was significantly upregulated following starvation at both mRNA and protein levels. Inhibition of Annexin A1 expression with specific siRNA did not alter starvation-induced autophagy as measured by the level of lipidated LC3, but significantly reversed autophagy degradation as measured by the level of p62/SQSTM 1. Thus Annexin A1 seemed to be positively upregulated during starvation to promote autophagic degradation. Overall, the data presented in this study established a expression profile of the proteome in starved cells, which allowed the identification of proteins with potential significance in starvation-induced autophagy.     

Ubiquitin ligase substrate identification through quantitative proteomics at both the protein and peptide levels

Protein ubiquitination is a key regulatory process essential to life at a cellular level; significant efforts have been made to identify ubiquitinated proteins through proteomics studies, but the level of success has not reached that of heavily studied post-translational modifications, such as phosphorylation. HRD1, an E3 ubiquitin ligase, has been implicated in rheumatoid arthritis, but no disease-relevant substrates have been identified. To identify these substrates, we have taken both peptide and protein level approaches to enrich for ubiquitinated proteins in the presence and absence of HRD1. At the protein level, a two-step strategy was taken using cells expressing His(6)-tagged ubiquitin, enriching proteins first based on their ubiquitination and second based on the His tag with protein identification by LC-MS/MS. Application of this method resulted in identification and quantification of more than 400 ubiquitinated proteins, a fraction of which were found to be sensitive to HRD1 and were therefore deemed candidate substrates. In a second approach, ubiquitinated peptides were enriched after tryptic digestion by peptide immunoprecipitation using an antibody specific for the diglycine-labeled internal lysine residue indicative of protein ubiquitination, with peptides and ubiquitination sites identified by LC-MS/MS. Peptide immunoprecipitation resulted in identification of over 1800 ubiquitinated peptides on over 900 proteins in each study, with several proteins emerging as sensitive to HRD1 levels. Notably, significant overlap exists between the HRD1 substrates identified by the protein-based and the peptide-based strategies, with clear cross-validation apparent both qualitatively and quantitatively, demonstrating the effectiveness of both strategies and furthering our understanding of HRD1 biology.     

表阿霉素对肝癌亚细胞蛋白质组影响的研究

表阿霉素是临床应用的抗癌药物之一,它干扰DNA的复制和转录,并能诱导癌细胞凋亡。表阿霉素对癌细胞蛋白质表达谱的影响,涉及到表阿霉素药理作用的机制,对此进行研究,有助于抗癌药物作用机制的理解。运用亚细胞蛋白质组学的研究策略,对比分析表阿霉素干预前后,肝癌细胞线粒体、细胞核蛋白质组的表达差异。通过超离心分离纯化细胞器,双向电泳分离蛋白质,图像分析筛选差异表达蛋白,MALDI-TOF-MS分析鉴定蛋白质。从线粒体、细胞核两个亚细胞组分中一个鉴定了15种差异表达蛋白,其中5种在表阿霉素干预后的肝癌细胞中表达上调,10种干预后表达下调。这些差异表达蛋白质涉及到细胞的能量代谢、蛋白质合成、细胞骨架的改变、mRNA的加工成熟、细胞应急状态的形成及凋亡调控等许多方面。     

Characterization and quantitation of membrane proteomes using multidimensional MS-based proteomic technologies

A major goal of proteomics is to develop methods that enable the systematic characterization of every protein within the cell or particular subcellular proteome using a single analytical platform. Although the equivalent has already been achieved in genomics, reaching this goal in proteomics represents a much greater challenge due to the wide dynamic range of protein expression, numerous post-translational modifications and remarkable physicochemical heterogeneity of proteins. A major analytical challenge has involved developing more effective means for proteome-scale investigations of membrane proteins, whose solubility differs drastically from that of cytoplasmic proteins. Fortunately, rapid progress has increased the ability to characterize this critically important class of proteins on a scale analogous to that of aqueous soluble proteins.     

Comparison of Golgi apparatus and endoplasmic reticulum proteins from livers of juvenile and aged rats using a novel technique for separation and enrichment of organelles.

The broad dynamic range of protein abundances, which can vary from about 10(6) for cells to 10(10) for tissues in complex proteomes, continues to challenge proteomics research. Proteome analysis, in particular organelle proteomics, using current approaches, requires extensive fractionation, separation, and enrichment. Over the years, organelle separation was achieved through the use of differential and density-gradient ultracentrifugation. However, the traditional fixed-volume process is a time-consuming and labor-intensive method, especially with large quantities of sample. Here, we present a novel tool for subcellular fractionation of biologically complex mixtures: continuous-flow ultracentrifugation of tissue homogenates to obtain both organelle separation and extensive organelle enrichment at the same time. In this study, rat liver tissues from two different age groups (3-8 wk and greater than 1 y old) were homogenized by blending. After removing nuclei, the resulting homogenates were further fractionated at the subcellular level by the use of sucrose gradient continuous-flow ultracentrifugation. Each organelle's enriched fractions were identified by Western blot analysis. To study the possible effects of aging on the endoplasmic reticulum and Golgi apparatus, we compared the organelle protein profiles of the two groups of rat liver tissues using two-dimensional gel electrophoresis, digitized imaging of two-dimensional gel electrophoresis, and mass spectrometry. Significant differences in the protein profiles of both organelles were observed between the two groups of rat tissues. The technique described here for fractionation and enrichment of organelles demonstrated a useful tool for proteomics research, including identification of low-abundance proteins and post-translational modifications.     

Characterization and quantitation of membrane proteomes using multidimensional MS-based proteomic technologies

A major goal of proteomics is to develop methods that enable the systematic characterization of every protein within the cell or particular subcellular proteome using a single analytical platform. Although the equivalent has already been achieved in genomics, reaching this goal in proteomics represents a much greater challenge due to the wide dynamic range of protein expression, numerous post-translational modifications and remarkable physicochemical heterogeneity of proteins. A major analytical challenge has involved developing more effective means for proteome-scale investigations of membrane proteins, whose solubility differs drastically from that of cytoplasmic proteins. Fortunately, rapid progress has increased the ability to characterize this critically important class of proteins on a scale analogous to that of aqueous soluble proteins.