Proteomic analysis of membrane-associated proteins from rat liver autophagosomes

Proteins associated with membranes from purified rat liver autophagosomes were separated by two-dimensional (2D) gel electrophoresis (zoom gels, p14-7 and 6-9), silver-stained and identified by MALDI-TOF mass spectrometry. Among >1,500 detectable protein spots, 58 (derived from 39 different known proteins) were at least twofold (and significantly) enriched in autophagosomal membranes relative to cytoplasmic membranes. All of these membrane-associated proteins were also present in the cytosol, many of them being truncated enzyme variants that would be expected to serve a binding rather than an enzymatic function. Eleven proteins were highly enriched (consistent with the theoretical maximum of 25x), corresponding to an exclusive membrane localization in the delimiting membrane of the autophagosome. Three of these were methyltransferases: betaine:homocysteine methyltransferase (five variants); catechol O-methyltransferase (one phosphorylated and one unphosphorylated variant) and methionine adenosyltransferase, perhaps indicating that methylation/demethylation of membrane components could play a role in autophagy. A fourth highly enriched autophagosomal protein phosphatidylethanolamine binding protein, is particularly interesting considering that the autophagic marker protein, LC3/ Atg8, is linked to autophagosomal membranes through its covalent conjugation with phosphatidylethanolamine (as the form LC3-II). LC3-II was not detectable on silver-stained 2D-gels, but could be shown by immunoblotting to be highly enriched in autophagosomal membranes. Other highly enriched proteins were heat shock cognate protein Hsc70 (one short and one long variant), peroxiredoxin 2, peroxiredoxin 6 (two variants), fructose 1,6-bisphosphatase (one phosphorylated and one unphosphorylated variant), adenosine kinase, inorganic pyrophosphatase and selenium-binding protein 2. Hsc70, a chaperonin that plays an important role in the recognition and proteasomal degradation of aggregated proteins as well as in the lysosomal membrane uptake and degradation of certain cytosolic proteins (chaperone-mediated autophagy), could conceivably also serve a recognition function in the autophagic scavenging of denatured or aggregated proteins (aggrephagy). The moderately enriched (2-14x) autophagosomal membrane-associated proteins included a remarkably high proportion of drug-metabolizing enzymes, such as several glutathione S-transferases, sulfotransferases and aromatic hydrocarbon/steroid oxidoreductases. If the autophagic function of these proteins is to recognize protein-drug adducts, they may, along with the peroxiredoxins, chaperonins and methyl metabolic enzymes, make the phagophores (the sequestering precursors of the autophagosomal delimiting membrane) well equipped for the detection and scavenging of proteins denatured by oxidation, hypermethylation, drug adduction or other mechanisms.