A divalent interaction between HPS1 and HPS4 is required for the formation of the biogenesis of lysosome-related organelle complex-3 (BLOC-3)

► An HPS1 isoform encoded by a transcript lacking exon 9 interacts with HPS4 ► The central region of HPS4 protein interacts dually, with either the N- or C-terminus of HPS1 ► Interacting regions of HPS1 and HPS4 exist as soluble and membrane-associated forms ► Missense mutations within interacting regions of HPS1 alter its stability ► Membrane association of HPS1 and HPS4 depends upon specific regions of these proteins