The dopamine transporter/cocaine receptor (DAT) is the site at which cocaine exerts rewarding/reinforcing effects. Cloning of a rat DAT cDNA provided primary sequence information and cDNA clones that facilitated production of antipeptide- and antiprotein antibodies that could allow detection of the DAT protein immunoreactivity. Determination of the DAT primary structure still left open questions about possible post-translational modifications of this protein, and the roles that they might play in transporter regulation and substrate or cocaine recognition. Interest in the human protein, and finding through cDNA cloning that the human protein differed at sites for N-linked glycosylation, also motivated interest in the species differences and postmortem stability of the transporter. Conjugated peptide preparations and the N-terminal DAT biosynthesized fusion protein fragment elicited antibody responses. One antipeptide antibody immunoprecipitates photoaffinity-labeled transporter, and recognizes a band consistent with the transporter's apparent molecular weight of 68 kD in Western analyses. Photoaffinity labeling studies reveal differences in the molecular masses of the rat and human dopamine transporter that are consistent with the different extents of glycosylation predicted from primary sequence. However, the human transporter may also undergo postmortem degradation, since a rat model of this process reveals a diminishing molecular mass with time after death. DAT cDNAs expressed in COS cells, although binding with high affinity and appropriate pharmacological profile, nevertheless demonstrate a higher molecular mass and less susceptibility to neuraminidase than the native protein. The cloned and expressed DAT is thus subject to at least as much posttranslational modification by glycosylation as the wild-type protein, although the sugar composition may be different. The molecule is also phosphorylated; the N-terminal fusion protein fragment can be phosphorylated by protein kinase A.
