STX13 regulates cargo delivery from recycling endosomes during melanosome biogenesis

Abstract

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, also known as STX12), a recycling endosomal Qa-SNARE, inhibits pigment granule maturation in melanocytes by rerouting the melanosomal proteins such as TYR and TYRP1 to lysosomes. Furthermore, live-cell imaging and electron microscopy studies showed that STX13 co-distributed with melanosomal cargo in the tubular-vesicular endosomes that are closely associated with the maturing melanosomes. STX family proteins contain an N-terminal regulatory domain, and deletion of this domain in STX13 increases both the SNARE activity in vivo and melanosome cargo transport and pigmentation, suggesting that STX13 acts as a fusion SNARE in melanosomal trafficking pathways. In addition, STX13-dependent cargo transport requires the 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.

Rab22A recruits BLOC ‐1 and BLOC ‐2 to promote the biogenesis of recycling endosomes

“…These findings suggest that Rab22A regulates the T N and T L of KIF13A‐positive REs similar to the MHC‐1‐positive tubules shown previously . Additionally, subcellular membrane fractionation showed that Rab22A was enriched in the membrane (1–1.4 M sucrose) fractions positive for the RE markers STX13 , KIF13A and AP‐1 (Fig D). As a control, the membrane fractions were also probed for other organelle‐specific makers such as GM130 ( cis ‐Golgi), LIMPII (LE and TGN) and LAMP‐2 (lysosome) (Fig D).…”

“…This was supported by live cell imaging experiments, where a cohort of TYRP1‐GFP localized to the RFP‐STX13‐positive vacuolar endosomes in Rab22A‐depleted melanocytes (Fig EV4A). As expected, TYRP1‐GFP in control melanocytes localized majorly to the melanosomes while a small cohort to the RFP‐STX13‐positive endosomes or tubular structures (Fig EV4A) similar to other melanocytic cargoes . Immunoblotting analyses showed that TYRP1, ATP7A and TYR (tyrosinase) protein levels were dramatically reduced, while the proteolytic processing of PMEL (full length (P1) into Mβ) was moderately affected in Rab22A‐depleted melanocytes compared to control cells (Fig C).…”

“…In contrast to the HeLa cells, mCherry‐Rab22A WT in wild‐type mouse melanocytes (melan‐Ink) localizes to STX13‐positive enlarged vacuolar endosomes and also to the buds (arrowheads, persisted for < 20 s in live cell imaging) emanating from these endosomes in live cell imaging experiments. However, mCherry‐Rab22A WT does not appear as extended tubular structures as seen in HeLa cells (Fig EV2A and Movie EV4).…”

Evolution of heterogeneous genome differentiation across multiple contact zones in a crow species complex

“…2b). It contained 16 genes, 1 of which, STX12, affects pigmentation by involvement in melanosome trafficking34. LRP5 , described above for the cornix – orientalis contact zone, was located in an outlier window and also associated with divergent cacti (Fig.…”

Phosphoproteomics reveals that the hVPS34 regulated SGK3 kinase specifically phosphorylates endosomal proteins including Syntaxin-7, Syntaxin-12, RFIP4 and WDR44

“…STX12 is localized in early and recycling endosomes as well as the plasma membrane, and interacts with the t-SNARE proteins syntaxin 6 and Vti1a and the v-SNARE protein VAMP4 and the formed complex is responsible for the homotypic fusion of early endosomes [51,52]. Furthermore, it has been suggested that STX12 can function at the plasma membrane and can regulate membrane fusion and recycling of plasma membrane proteins [63]. The N-terminal Habc regulatory interacts intramolecularly with the SNARE domain, rendering the protein to a "closed conformation" and prohibiting the interaction with the cognate partners and the assembly of the SNARE complex [64][65][66].…”