Coimmunoprecipitation of HA₃-RP and BLOC-1. Whole cell extracts of metabolically labeled MNT-1 cells stably expressing HA₃-RP were subjected to immunoprecipitation-recapture analysis, as described in the legend of Figure 4. Rabbit antisera to either irrelevant proteins (C, controls), the σ subunit of AP-3 (σ3), pallidin (PA), the Hermansky-Pudlak syndrome 1 (HPS1) protein, the Hermansky-Pudlak syndrome 4 (HPS4) protein, and mouse monoclonal anti-myc (control) or anti-HA were used in the 1st IP. Mouse monoclonal anti-HA was used in the 2nd IP step. To confirm the presence of AP-3, BLOC-1, and BLOC-3 complexes after the first immunoprecipitation (1st IP), the supernatant obtained after the recapture step (2nd IP) were subjected to 2 additional IP steps (3rd IP and 4th IP) using antibodies to σ3 and μ3, pallidin and cappuccino, or HPS1 and HPS4 or HPS4 and HPS1 in the consecutive IP steps (3rd IP and 4th IP), respectively. The positions of molecular mass markers are indicated on the right.