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The human brain mannose 6‐phosphate glycoproteome: A complex mixture composed of multiple isoforms of many soluble lysosomal proteins

The lysosome is a membrane delimited cytoplasmic organelle that contains at least 50 hydrolytic enzymes and associated cofactors. The biomedical importance of these enzymes is highlighted by the many lysosomal storage disorders that are associated with mutations in genes encoding lysosomal proteins, and there is also evidence that lysosomal activities may be involved in more widespread human diseases. The aim of this study was to characterize the human brain lysosomal proteome with the goal of establishing a reference map to investigate human diseases of unknown etiology and to gain insights into the cellular function of the lysosome. Proteins containing mannose 6‐phosphate (Man6‐P), a carbohydrate modification used for targeting resident soluble lysosomal proteins to the lysosome, were affinity‐purified using immobilized Man6‐P receptor. Fractionation by two‐dimensional electrophoresis resolved a complex mixture comprising approximately 800 spots. Constituent proteins in each spot were identified using a combination of matrix‐assisted laser desorption/ionization‐time of flight mass spectrometry (both mass spectrometry (MS/MS) and tandem peptide mass fingerprinting) on in‐gel tryptic digests and N‐terminal sequencing. In a complementary analysis, we also analyzed a tryptic digest of the unfractionated mixture by liquid chromatography MS/MS. In total, 61 different proteins were identified. Seven were likely contaminants associated with true Man6‐P glycoproteins. Forty‐one were known lysosomal proteins of which 11 have not previously been reported to contain Man6‐P. An additional nine proteins were either uncharacterized or proteins not previously reported to have lysosomal function. We found that the human brain Man6‐P‐containing lysosomal proteome is highly complex and contains more proteins with a much greater number of individual isoforms than found in previous studies of Man6‐P glycoproteomes.