Constitutive high‐affinity choline transporter endocytosis is determined by a carboxyl‐terminal tail dileucine motif

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Abstract
Maintenance of acetylcholine synthesis depends on the effective functioning of a high‐affinity sodium‐dependent choline transporter (CHT1). Recent studies have shown that this transporter is predominantly localized inside the cell, unlike other neurotransmitter transporters, suggesting that the trafficking of CHT1 to and from the plasma membrane may play a crucial role in regulating choline uptake. Here we found that CHT1 is rapidly and constitutively internalized in clathrin‐coated vesicles to Rab5‐positive early endosomes. CHT1 internalization is controlled by an atypical carboxyl‐terminal dileucine‐like motif (L531, V532) which, upon replacement by alanine residues, blocks CHT1 internalization in both human embryonic kidney 293 cells and primary cortical neurons and results in both increased CHT1 cell surface expression and choline transport activity. Perturbation of clathrin‐mediated endocytosis with dynamin‐I K44A increases cell surface expression and transport activity to a similar extent as mutating the dileucine motif, suggesting that we have identified the motif responsible for constitutive CHT1 internalization. Based on the observation that the localization of CHT1 to the plasma membrane is transient, we propose that acetylcholine synthesis may be influenced by processes that lead to the attenuation of constitutive CHT1 endocytosis.