The Organelle Proteome of the DT40 Lymphocyte Cell Line

A major challenge in eukaryotic cell biology is to understand the roles of individual proteins and the subcellular compartments in which they reside. Here, we use the localization of organelle proteins by isotope tagging technique to complete the first proteomic analysis of the major organelles of the DT40 lymphocyte cell line. This cell line is emerging as an important research tool because of the ease with which gene knockouts can be generated. We identify 1090 proteins through the analysis of preparations enriched for integral membrane or soluble and peripherally associated proteins and localize 223 proteins to the endoplasmic reticulum, Golgi, lysosome, mitochondrion, or plasma membrane by matching their density gradient distributions to those of known organelle residents. A striking finding is that within the secretory and endocytic pathway a high proportion of proteins are not uniquely localized to a single organelle, emphasizing the dynamic steady-state nature of intracellular compartments in eukaryotic cells.The chicken pre-B cell line DT40 exhibits a remarkably high ratio of targeted to random integration for transfected DNA constructs. This property is unusual in vertebrate cell lines and enables targeted gene disruption experiments to be carried out with relative ease (1). Consequently, DT40 has become a major research tool for the molecular dissection of a wide range of cellular and biochemical mechanisms in a vertebrate context, including membrane traffic, signal transduction, and cell cycle (2).Proteins in eukaryotic cells are organized according to their functions within a dynamic network of membranes. Localization is therefore paramount in assigning functions to uncharacterized proteins and understanding the processes occurring in subcellular compartments. An increased knowledge of the protein localization within the DT40 cell line would be of great value. Traditional localization methods such as immunofluorescence microscopy are typically low throughput and are more suitably applied to the study of specific proteins of interest rather than the cataloguing of large numbers of proteins. Recent developments in proteomics have made it possible to analyze the protein composition of organelles using a variety of different approaches. Several groups have utilized label-free quantitative proteomics in the high throughput assignment of proteins to subcellular compartments. In one approach, protein correlation profiling, proteins from enriched organelle fractions are quantified by peptide ion intensity measurements (3, 4). Other similar methods employ quantitation by spectral counting, recording the number of ions detected per protein (5, 6). Localization of organelle proteins by isotope tagging (LOPIT)¹ is a complementary approach, which employs isotope labeling for quantitation (7–9). Rather than processing each sample separately as in label-free techniques, differentially labeled fractions are pooled early in the LOPIT protocol. This has the important advantage of reducing the points at which variation might be introduced into the data.LOPIT begins with the partial separation of organelles by density gradient centrifugation and relies on the assumption that proteins from each organelle co-fractionate. Protein profiles along the gradient are quantified by the use of isotopically coded tags in conjunction with two-dimensional liquid chromatography of peptides and tandem mass spectrometry. Multivariate statistical techniques are then used to assign localizations to proteins by comparing their gradient profiles to those of established organelle markers in an unbiased manner. The major strength of such an approach is that it enables residents of different subcellular compartments to be resolved even if their gradient distributions overlap, and genuine organelle constituents can be readily distinguished from contaminants.Here we use LOPIT to produce the first proteomic analysis of the major organelles of DT40. We have reproducibly identified 1090 proteins through the parallel analysis of preparations enriched for integral membrane or soluble and peripherally associated proteins. We use the distributions of 102 known organelle resident proteins as a basis to assign a further 223 proteins to five organelles: 79 to the endoplasmic reticulum (ER), 42 to the Golgi, 2 to the lysosome, 31 to the mitochondrion, and 69 to the plasma membrane (PM). We also demonstrate the resolution of components of the vesicular transport machinery. A striking finding is that a high proportion of identified proteins are not localized to a single organelle. This indicates that at steady state a substantial fraction of proteins are in transit between compartments, emphasizing the dynamic nature of intracellular organelles in eukaryotic cells. Our results represent the first application of LOPIT to a vertebrate system, provide the first organelle proteomic analysis of any lymphocyte cell line, and establish a major resource for the DT40 community.