D283 Med, a Cell Line Derived from Peritoneal Metastatic Medulloblastoma: A Good Choice for Missing Protein Discovery.

Since the Chromosome-Centric Human Proteome Project (C-HPP) was launched in 2010, many techniques have been adopted for the discovery of missing proteins (MPs). Because of these efforts, only 1481 MPs remained as of July 2020; however, by relying only on technique optimization, researchers have reached a bottleneck in MP discovery. Protein expression is tissue- or cell-type-dependent. The tissues of the human testis and brain have been reported to harbor a large number of tissue-specific genes and proteins; however, few studies have been performed on human brain tissue or cells to identify MPs. Herein a metastatic cell line derived from brain cancer, D283 Med, was used to search for MPs. With a traditional and simple shotgun workflow to separate the peptides into 20 fractions, 12 MPs containing at least two unique non-nested peptides (amino acid length ≥9) were identified in this cell line with a protein false discovery rate of <1%. Following the same experimental protocol, only one MP was found in a nonmetastatic brain cancer cell line, U-118 MG. Furthermore, 12 MPs were verified as having two non-nested unique peptides by matching them with corresponding chemically synthesized peptides through parallel reaction monitoring. These results clearly demonstrate that the appropriate selection of experimental materials, either tissues or cell lines, is imperative for MP discovery. The data obtained in this study are available via ProteomeXchange (PXD021482) and PeptideAtlas (PASS01627).