STX13 regulates cargo delivery from recycling endosomes during melanosome biogenesis

Melanosomes are a class of lysosome-related organelles produced by melanocytes. Biogenesis of melanosomes requires the transport of melanin synthesizing enzymes from tubular recycling endosomes to maturing melanosomes. The SNARE proteins involved in these transport or fusion steps have been poorly studied. We found that depletion of syntaxin 13 (STX13), a recycling endosomal Qa-SNARE, inhibits pigment granule maturation in melanocytes by rerouting the melanosomal proteins such as TYR and TYRP1 to lysosomes. Further, live cell imaging and electron microscopy studies showed STX13 co-distributed with melanosomal cargo in the tubular-vesicular endosomes that are closely associated with the maturing melanosomes. STX family proteins contain N-terminal regulatory domain, deletion of this domain in STX13 raises the SNARE activity in vivo and increases the melanosome cargo transport and pigmentation, suggesting that STX13 acts as a fusion SNARE in the melanosomal trafficking pathways. In addition, STX13-dependent cargo transport requires a melanosomal R-SNARE, VAMP7 and its silencing blocks the melanosome maturation, reflecting a defect in endosome-melanosome fusion. Moreover, we show mutual dependency between STX13 and VAMP7 in regulating their localization for efficient cargo delivery to melanosomes.