Abstract
Rat C6 glioma cells were used as a model system to study the biosynthesis, intracellular targeting, and degradation of the EAAC1 transporter, a sodium-dependent glutamate/aspartate transport protein that encodes System X(-)A,G activity. At steady state, nearly 70% of the EAAC1 transporter was located at the cell surface. The newly synthesized EAAC1 protein was co-translationally N-glycosylated with high mannose oligosaccharide chains that were processed into complex-type sugar chains as the protein matured. The final maturation steps for EAAC1 protein coincided with its plasma membrane arrival, which was first detected at about 45 min after the initial synthesis. The newly synthesized EAAC1 protein was protected from degradation during the maturation and targeting process, as well as during the first 5 h after plasma membrane arrival. After this initial lag period, both the newly synthesized transporter and the total cellular EAAC1 pool were degraded by first order kinetics with a half-life of 6 h. These results represent the first analysis of the synthesis and degradation of the EAAC1 amino acid transporter.
Highlights
A family of Naϩ-dependent high affinity glutamate/aspartate transport systems, previously referred to collectively as System XϪA,G, is essential for the glutamatergic transmission in the central nervous system [1,2,3], as well as for nutrition of many other cells
Rat C6 glioma cells were used as a model system to study the biosynthesis, intracellular targeting, and degradation of the EAAC1 transporter, a sodium-dependent glutamate/aspartate transport protein that encodes System X؊A,G activity
EAAC1 Localization at the Cell Surface—To demonstrate the plasma membrane localization of EAAC1 transporter in C6 cells, the accessibility of EAAC1 protein from the extracellular space was tested by cell surface biotinylation using a membrane-impermeable sulfo-NHS-LC-biotin reagent
Summary
A family of Naϩ-dependent high affinity glutamate/aspartate transport systems, previously referred to collectively as System XϪA,G, is essential for the glutamatergic transmission in the central nervous system [1,2,3], as well as for nutrition of many other cells. Rat EAAC1 cDNA encodes an integral membrane protein of 523 amino acids with a predicated nonglycosylated core molecular mass of 56.8 kDa [10], but it is N-glycosylated under normal conditions [11]. Rat C6 glioma cells exhibit several biochemical features of normal glial cells, such as expressing glial fibrillary acidic protein [15]. These cells express a high level of System XϪA,G transport activity [16]. The newly synthesized plasma membrane-associated transporter population was degraded at the same rate as the total pool of EAAC1, suggesting that both surface and intracellular EAAC1 proteins have similar half-life values. The results obtained provide the basis for studying the role of EAAC1 transporter synthesis and degradation during disease states
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