C. elegans BLOC-1 Functions in Trafficking to Lysosome-Related Gut Granules

Abstract

The human disease Hermansky-Pudlak syndrome results from defective biogenesis of lysosome-related organelles (LROs) and can be caused by mutations in subunits of the BLOC-1 complex. Here we show that C. elegans glo-2 and snpn-1, despite relatively low levels of amino acid identity, encode Pallidin and Snapin BLOC-1 subunit homologues, respectively. BLOC-1 subunit interactions involving Pallidin and Snapin were conserved for GLO-2 and SNPN-1. Mutations in glo-2 and snpn-1,or RNAi targeting 5 other BLOC-1 subunit homologues in a genetic background sensitized for glo-2 function, led to defects in the biogenesis of lysosome-related gut granules. These results indicate that the BLOC-1 complex is conserved in C. elegans. To address the function of C. elegans BLOC-1, we assessed the intracellular sorting of CDF-2::GFP, LMP-1, and PGP-2 to gut granules. We validated their utility by analyzing their mislocalization in intestinal cells lacking the function of AP-3, which participates in an evolutionarily conserved sorting pathway to LROs. BLOC-1(−) intestinal cells missorted gut granule cargo to the plasma membrane and conventional lysosomes and did not have obviously altered function or morphology of organelles composing the conventional lysosome protein sorting pathway. Double mutant analysis and comparison of AP-3(−) and BLOC-1(−) phenotypes revealed that BLOC-1 has some functions independent of the AP-3 adaptor complex in trafficking to gut granules. We discuss similarities and differences of BLOC-1 activity in the biogenesis of gut granules as compared to mammalian melanosomes, where BLOC-1 has been most extensively studied for its role in sorting to LROs. Our work opens up the opportunity to address the function of this poorly understood complex in cell and organismal physiology using the genetic approaches available in C. elegans.

BORC, a Multisubunit Complex that Regulates Lysosome Positioning

“…Perhaps BORC is more important than BLOC-1 or both complexes have partly overlapping functions in supporting viability. C. elegans Snapin mutants are viable but exhibit defects in the biogenesis of gut granules (a type of LRO) (Hermann et al, 2012) and the synaptic vesicle cycle (Yu et al, 2013). …”

The Intestinal Copper Exporter CUA-1 Is Required for Systemic Copper Homeostasis in Caenorhabditis elegans

“…One possible sorting complex is the biogenesis of lysosome-related organelles complex-1 (BLOC-1), which is a known regulator of intracellular trafficking to lysosome-related organelles in mammals, Drosophila, and C. elegans (39 -41). ATP7A is known to supply copper to melanosomes in a BLOC-1-dependent manner in mammalian cells, and BLOC-1 subunits are also required for the proper trafficking of gut granule cargo in worms (39,42). Given that CUA-1.1 localizes to gut granules and that ATP7A localizes to melanosomes in response to elevated levels of copper, redistribution of the copper exporter may require the BLOC-1 complex and related sorting proteins in metazoans.…”

C. elegans as a model for membrane traffic

“…glo-2 encodes a pallidin/HSP-9 homologue, which is a subunit of the BLOC-1 complex (Hermann et al, 2012; PMID 22916203). GLO-2::GFP driven by the own promoter was expressed in the intestinal cells and localized in the cytoplasm.…”

“…Mutations in the components of these complexes are associated with Hermansky-Pudlak syndrome (Cullinane et al, 2011; PMID 21665000; Huizing et al, 2008; PMID 18544035; Sitaram et al, 2012; PMID 22718909). The C. elegans genome encodes 7 BLOC-1 subunits including GLO-2, BLOS-1, BLOS-2, BLOS-4, DSBN-1, MUTD-1, and SNPN-1, but seems to lack a BLOS-3 homolog (Hermann et al, 2012; PMID 22916203). In yeast two-hybrid assay, GLO-2 interacts with itself, BLOS-1 and BLOS-4 while SNPN-1 interacts with DSBN-1, suggesting that BLOC-1 components form complexes in C. elegans as they do in other species (Hermann et al, 2012; PMID 22916203).…”