Differential glycosylation and intracellular trafficking for the long and short isoforms of the D2 dopamine receptor.

The D2 dopamine receptor exists in two alternatively spliced isoforms, "long" and "short" (D2L and D2S), which differ by 29 amino acids in the third cytoplasmic domain. The functional differences between these two isoforms are still obscure. We have performed pulse-chase studies on the D2L and D2S receptors expressed in CHO cells in order to follow the post-translational processing of the two isoforms. Both isoforms are present in three post-translational states: a newly synthesized protein, a partially glycosylated product, and a fully glycosylated mature 70-kDa receptor. However, the processing to the mature receptor differs between the two isoforms. First, the D2S receptor is processed to the mature 70-kDa species faster than the D2L receptor. Second, at 20 degrees C the D2S isoform is fully processed to the 70-kDa species, whereas the D2L isoform persists in its partially processed 45-kDa state. Finally, a significant portion of the D2L receptor remains in its partially processed form in an intracellular compartment and does not reach the plasma membrane. These results give rise to the suggestion that the difference observed between the two alternatively spliced isoforms of the D2 receptor may lie in their post-translational processing and intracellular trafficking.