The roles of Syx5 in Golgi morphology and Rhodopsin transport inDrosophilaphotoreceptors

Abstract

SNAREs (SNAP receptors) are the key components of protein complexes that drive membrane fusion. Here, we report the function of a SNARE, Syntaxin 5 (Syx5), in the development of photoreceptors in Drosophila. In wild-type photoreceptors, Syx5 localizes to cis-Golgi, along with cis-Golgi markers: Rab1 and GM130. We observed that Syx5-deficient photoreceptors show notable accumulation of these cis-Golgi markers accompanying drastic accumulation of vesicles between endoplasmic reticulum (ER) and Golgi cisternae. Extensive analysis of Rh1 (rhodopsin 1) trafficking revealed that in Syx5-deficient photoreceptors, Rh1 is exported from the ER with normal kinetics, retained in the cis-Golgi region along with GM130 for a prolonged period, and then subsequently degraded presumably by endoplasmic reticulum-associated protein degradation (ERAD) after retrieval to the ER. Unlike our previous report of Rab6-deficient photoreceptors – where two apical transport pathways are specifically inhibited – vesicle transport pathways to all plasma membrane domains are inhibited in Syx5-deficient photoreceptors, implying that Rab6 and Syx5 are acting in different steps of intra-Golgi transport. These results indicate that Syx5 is crucial for membrane protein transport, presumably during ER-derived vesicle fusion to form cis-Golgi cisternae.

The two TRAPP complexes of metazoans have distinct roles and act on different Rab GTPases

“…However, it is also possible that TRAPPC11 allows the complex to have an additional activity that budding yeast no longer requires. Our finding that in some tissues, TRAPPIII is found on the trans-Golgi as well as the cis-Golgi provides some support for this possibility as Rab1 is widely believed to function only on the cis-Golgi ( Satoh et al, 2005 , 2016 ), and consistent with this, we could only detect YFP-Rab1 on the cis-Golgi in these tissues (Fig. S5).…”

Parcas is the predominant Rab11-GEF for rhodopsin transport in Drosophila photoreceptors

“…The immunostaining of Na + K + ATPase at the basolateral membrane was also reduced. These phenotypes were similar to those caused by the loss of Syx5, which regulates ER-to-Golgi transport, and these findings are in agreement with those of a recent study that showed that fly TRAPPIII complex is a Rab1-GEF (Riedel et al, 2018;Satoh et al, 2016). We next investigated the influence of TRAPPIII deficiency on Rab1 and Rab11 localization.…”

A Rab11-GEF Parcas recruits Rab11 onto recycling endosomes for rhodopsin transport in Drosophila photoreceptors

“…The immunostaining of Na + K + ATPase at 265 the basolateral membrane was also reduced. These phenotypes were similar to those 266 caused by the loss of Syx5, which regulates ER to Golgi transport, and these findings 267 are in agreement with those of a recent study that showed that fly TRAPPIII is a Rab1-268 GEF (Riedel et al, 2018;Satoh et al, 2016). We next investigated the influence of 269 TRAPPIII-deficiency on Rab1 and Rab11 localization.…”