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The cellular slime mold Dictyostelium discoideum is a soil-living eukaryote, which feeds on microorganisms engulfed by phagocytosis. Axenic laboratory strains have been produced that are able to use liquid growth medium internalized by macropinocytosis as the source of food. To better define the macropinocytosis process, we established the inventory of proteins associated with this pathway using mass spectrometry-based proteomics. Using a magnetic purification procedure and high-performance LC-MS/MS proteome analysis, a list of 2108 non-redundant proteins was established, of which 24% featured membrane-spanning domains. Bioinformatics analyses indicated that the most abundant proteins were linked to signaling, vesicular trafficking and the cytoskeleton. The present repertoire validates our purification method and paves the way for a future proteomics approach to study the dynamics of macropinocytosis.

Multiplexed tandem mass spectrometry (MS/MS) has recently been demonstrated as a means to increase the throughput of peptide identification in liquid chromatography (LC) MS/MS experiments. In this approach, a set of parent species is dissociated simultaneously and measured in a single spectrum (in the same manner that a single parent ion is conventionally studied), providing a gain in sensitivity and throughput proportional to the number of species that can be simultaneously addressed. In the present work, simulations performed using the Caenorhabditis elegans predicted proteins database show that multiplexed MS/MS data allow the identification of tryptic peptides from mixtures of up to ten peptides from a single dataset with only three "y" or "b" fragments per peptide and a mass accuracy of 2.5 to 5 ppm. At this level of database and data complexity, 98% of the 500 peptides considered in the simulation were correctly identified. This compares favorably with the rates obtained for classical MS/MS at more modest mass measurement accuracy. LC multiplexed Fourier transform-ion cyclotron resonance MS/MS data obtained from a 66 kDa protein (bovine serum albumin) tryptic digest sample are presented to illustrate the approach, and confirm that peptides can be effectively identified from the C. elegans database to which the protein sequence had been appended.