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The N-terminal tail of the glycine transporter: role in transporter phosphorylation.

In the vertebrate nervous system, glycinergic neurotransmission is tightly regulated by the action of the glycine transporters 1 and 2 (GlyT1 and GlyT2). Unlike GlyT1, the GlyT2 is characterized by the presence of a cytosolic 201 amino acids N-terminal tail of unknown function. In the present study we stably expressed a set of GlyT2 N-terminal deletion mutants and characterized the effect on uptake, trafficking and PKC-dependent post-translational modifications. The deletion of the first 43, 109 or 157 amino acids did not affect the trafficking and maturation of GlyT2. Similarly, a chimeric protein replacing GlyT2 N-terminus by the corresponding tail of GlyT1b or deletion of the entire N-terminus domain (D201), were able to reach the plasma membrane but showed faster turnover compared to the wild-type. All of these mutant proteins appeared as a mature, glycosylated transporter and a lower protein band corresponding to the non-glycosylated transporter. Interestingly, glycine uptake and PKC-mediated endocytosis and further reduction of transport were not affected by the deletions of 43–157 amino acids. Moreover, the analysis of PKC-dependent post-translational modifications for the mutants demonstrated that GlyT2 transporter ubiquitination was not affected; however, PKC-dependent transporter phosphorylation was completely abolished in the deletions of 157 and 201 amino acids. Altogether, these results suggest that the GlyT2 N-terminus does not directly participate in the modulation of transport properties but it could be relevant for scaffold formation and retention at active zones or communication to an intracellular signaling pathway.