Recent studies have led to the identification of a group of genes required for normal biogenesis of lysosome-related organelles such as melanosomes and platelet-dense granules. Two of these genes, which are defective in the pallid and muted mutant mouse strains, encode small, coiled-coil-forming proteins that display no homology to each other or to any known protein. We report that these two proteins, pallidin and muted, are components of a novel protein complex. We raised antibodies that allow for detection of pallidin from a wide variety of mammalian cells. Endogenous pallidin was distributed in both soluble and peripheral membrane protein fractions. Size-exclusion chromatography and sedimentation velocity analyses indicated that the bulk of cytosolic pallidin is a component of an asymmetric protein complex with a molecular mass of approximately 200 kDa. We named this complex BLOC-1 (for biogenesis of lysosome-related organelles complex 1). Steady-state pallidin protein levels were reduced in fibroblasts derived from muted and reduced pigmentation mice, suggesting that the genes defective in these two mutant strains could encode components of BLOC-1 that are required for pallidin stability. Co-immunoprecipitation and immunodepletion experiments using an antibody to muted confirmed that this protein is a subunit of BLOC-1. Yeast two-hybrid analyses revealed that pallidin is capable of self-association through a region that contains its two coiled-coil forming domains. Unlike AP-3-deficient pearl fibroblasts, which display defects in intracellular zinc storage, zinc distribution was not noticeably affected in pallid or muted fibroblasts. Interestingly, immunofluorescence and in vitro binding experiments demonstrated that pallidin/BLOC-1 is able to associate with actin filaments. We propose that BLOC-1 mediates the biogenesis of lysosome-related organelles by a mechanism that may involve self-assembly and interaction with the actin cytoskeleton.
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