C Core Vacuole/endosome Tethering Research Articles

Sec1/Munc18 protein Vps33 binds to SNARE domains and the quaternary SNARE complex

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins catalyze membrane fusion events in the secretory and endolysosomal systems, and all SNARE-mediated fusion processes require cofactors of the Sec1/Munc18 (SM) family. Vps33 is an SM protein and subunit of the Vps-C complexes HOPS (homotypic fusion and protein sorting) and CORVET (class C core vacuole/endosome tethering), which are central regulators of endocytic traffic. Here we present biochemical studies of interactions between Saccharomyces cerevisiae vacuolar SNAREs and the HOPS holocomplex or Vps33 alone. HOPS binds the N-terminal H(abc) domain of the Qa-family SNARE Vam3, but Vps33 is not required for this interaction. Instead, Vps33 binds the SNARE domains of Vam3, Vam7, and Nyv1. Vps33 directly binds vacuolar quaternary SNARE complexes, and the affinity of Vps33 for SNARE complexes is greater than for individual SNAREs. Through targeted mutational analyses, we identify missense mutations of Vps33 that produce a novel set of defects, including cargo missorting and the loss of Vps33-HOPS association. Together these data suggest a working model for membrane docking: HOPS associates with N-terminal domains of Vam3 and Vam7 through Vps33-independent interactions, which are followed by binding of Vps33, the HOPS SM protein, to SNARE domains and finally to the quaternary SNARE complex. Our results also strengthen the hypothesis that SNARE complex binding is a core attribute of SM protein function.

Identification of genes that enhance cellulase protein production in yeast

In order to enhance heterologous cellulase protein production in yeast, a plasmid harboring the endoglucanase gene from Clostridium thermocellum ( Ctcel8A) was used to systematically transform a homozygous diploid yeast deletion strain collection. We identified 55 deletion strains that exhibited enhanced endoglucanase activity compared with that of the wild-type strain. Genes disrupted in these strains were classified into the categories of transcription, translation, phospholipid synthesis, endosome/vacuole function, ER/Golgi function, nitrogen starvation response, and cytoskeleton. The vps3Δ and vps16Δ strains, which have deletion in genes encoding components of the class C core vacuole/endosome tethering (CORVET) complex, also exhibited enhanced β-glucosidase activity when Ctcel8A was heterologously expressed. Moreover, multiple gene deletion strains were constructed by using the vps3Δ strain. Endoglucanase activity of the resulting rav1Δ vps3Δ double deletion strain was exhibited higher than that of the rav1Δ or vps3Δ strains. Our genome-wide analyses using the yeast deletion strain collection identified useful genes that allow efficient expression of cellulase.