One interpretation of various band sizes is that DRD1 is forming a heterodimer with another DAR subtype or even another GPCR.

I believe the best explanation of the different bands found on western blots for Dopamine D1 is found in the paper entitled "D1 dopamine receptor signaling involves caveolin-2 in HEK-293 cells" by our collaborators in the Jose Lab. We find the similar set of bands from human renal proximal tubule cells. The bottom line is that the protein is N-glycosylated in at least 3 sites, palmitoylated in one site, and has as many as 15 serine threonine phosphorylation sites. We were so frustrated with the antibodies that we made our own polyclonal. Some lots of Santa Cruz rabbit polyclonal antibodies are even better than ours but they are hit or miss. Epitomics makes a rabbit monoclonal that works very well in our hands, and Cell Signalling worked with us to make two other rabbit monoclonals that work really well, but I don't know when they will be available. I hope that is helpful.

Thanks Josh and John. Will follow up and look up all the references in the paper you have quoted John and get back if I develop some insights :)!

I also meant to comment on what Josh had said above. DRD1 definitely heterodimerizes with other GPCRs. It also homodimerizes, and some of these complexes are resistant to full denaturation in loading buffer, and does add to the complexity of the banding seen on western. We were thinking at one time to use these extra bands as proof for a novel heterodimerization partner we found. Basically, I am saying I agree with what Josh was mentioning.