Golgi SM protein Sly1 promotes productivetrans-SNARE complex assembly through multiple mechanisms

Abstract

SNARE chaperones of the Sec1/mammalian Unc-18 (SM) family have critical roles in SNAREmediated membrane fusion. Using SNARE and Sly1 mutants, and a new in vitro assay of fusion, we separate and assess proposed mechanisms through which Sly1 augments fusion: (i) opening the closed conformation of the Qa-SNARE Sed5; (ii) close-range tethering of vesicles 5 to target organelles, mediated by the Sly1-specific regulatory loop; and (iii) preferential nucleation of productive trans-SNARE complexes. We show that all three mechanisms are important and operate in parallel, and we present evidence that close-range tethering is particularly important for trans-complex assembly when cis-SNARE assembly is a competing process. In addition, the autoinhibitory N-terminal Habc domain of Sed5 has at least two 10 positive activities: the Habc domain is needed for correct Sed5 localization, and it directly promotes Sly1-dependent fusion. Remarkably, "split Sed5," with the Habc domain present only as a soluble fragment, is functional both in vitro and in vivo.In addition to SNAREs and tethering factors, proteins of the Sec1/mammalian Unc-18 (SM) family have critical roles in SNARE-mediated fusion (Carr and Rizo, 2010;Rizo and Sudhof, 2012;Sudhof and Rothman, 2009). The first SM proteins identified through genetic 35 screens were Vps33a (carnation in Drosophila) and Saccharomyces cerevisiae Sec1 (UNC-18 in Caenorabditis elegans; Munc18-1 or nSec1 in mammals; Novick et al., 1979;Patterson, 1932).Despite their early identification and clear importance, and despite major efforts by many laboratories, the general mechanisms of SM function are only now emerging. All SM proteins exhibit strong evolutionary and structural homology, but they interact with cognate SNARE 40 proteins in very different ways. For example, yeast Sly1, yeast Vps45, and Munc18-1 all interact with short N-peptides at the amino termini of their cognate Qa-SNARE proteins (Bracher and Weissenhorn, 2002;Carpp et al., 2006;Dulubova et al., 2002;Furgason et al., 2009; Grabowski and Gallwitz, 1997;Yamaguchi et al., 2002). In contrast, Qa-SNARE N-peptide interactions do not occur with human or yeast Vps33, or with yeast Sec1 (Baker et al., 2015; Dulubova et al., 45 2001;Lobingier and Merz, 2012;Togneri et al., 2006). We have called SM proteins that interact with Qa-SNARE N-peptides Class I, and those that do not, Class II (Lobingier and Merz, 2012).Early structural and biochemical studies revealed that Munc18-1 tightly binds the Qa-SNARE Syntaxin-1A in its closed conformation, suggesting an inhibitory role for Munc18-1. (Dulubova et al., 1999;Misura et al., 2000;Yang et al., 2000). However, the emerging consensus 50 is that the core and evolutionarily conserved role of SM proteins is positive, rather than inhibitory. Specifically, SM proteins are hypothesized to nucleate and stabilize fusioncompetent trans-SNARE complexes (Carr and Rizo, 2010;Sudhof and Rothman, 2009;Toonen and Verhage, 2003;Yoon and Munson, 2018). A breakthrough was achieved in 2015 with two structure...

Recruitment of PI4KIIIβ to the Golgi by ACBD3 is dependent on an upstream pathway of a SNARE complex and golgins

“…SCFD1 (aka SLY1) is a soluble cytosolic Sec1/Munc18-like protein recruited to the cis -Golgi apparatus to participate in catalyzing the formation of a SNARE complex ( Linders et al. , 2019 ; Duan et al. , 2020a , 2020b ).…”

“…It has been proposed that SCFD1 interacts with the cis -Golgi SNARE STX5 which promotes the fusion of the incoming COPII vesicles by forming a SNARE complex with the v-SNARE SEC22B ( Duan et al. , 2020a , 2020b ).…”

Recruitment of PI4KIIIβ to the Golgi by ACBD3 is dependent on an upstream pathway of a SNARE complex and golgins

“…SCFD1 (aka SLY1) is a soluble cytosolic Sec1/Munc18-like protein recruited to the cis-Golgi apparatus to participate in catalysing the formation of a SNARE complex [45][46][47] . Exogenous overexpression of an SCFD1-HaloTag fusion colocalised with endogenous ACBD3 at the cis-Golgi apparatus (Figure 3A).…”

“…Thus, SCFD1 is upstream of ACBD3, which is upstream of PI4KIIIβ. It has been proposed that SCFD1 interacts with the cis-Golgi SNARE STX5 allowing it to switch to an open conformation by exposing its SNARE domain 46,47 . Then, SCFD1 bridges STX5 to the incoming COPII vesicles and promotes the formation of the SNARE complex between STX5 and the v-SNARE SEC22B, ultimately leading to membrane fusion.…”