SNARE Protein Snc1 Is Essential for Vesicle Trafficking, Membrane Fusion and Protein Secretion in Fungi

Abstract

Fungi are an important group of microorganisms that play crucial roles in a variety of ecological and biotechnological processes. Fungi depend on intracellular protein trafficking, which involves moving proteins from their site of synthesis to the final destination within or outside the cell. The soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) proteins are vital components of vesicle trafficking and membrane fusion, ultimately leading to the release of cargos to the target destination. The v-SNARE (vesicle-associated SNARE) Snc1 is responsible for anterograde and retrograde vesicle trafficking between the plasma membrane (PM) and Golgi. It allows for the fusion of exocytic vesicles to the PM and the subsequent recycling of Golgi-localized proteins back to the Golgi via three distinct and parallel recycling pathways. This recycling process requires several components, including a phospholipid flippase (Drs2-Cdc50), an F-box protein (Rcy1), a sorting nexin (Snx4-Atg20), a retromer submit, and the COPI coat complex. Snc1 interacts with exocytic SNAREs (Sso1/2, Sec9) and the exocytic complex to complete the process of exocytosis. It also interacts with endocytic SNAREs (Tlg1 and Tlg2) during endocytic trafficking. Snc1 has been extensively investigated in fungi and has been found to play crucial roles in various aspects of intracellular protein trafficking. When Snc1 is overexpressed alone or in combination with some key secretory components, it results in enhanced protein production. This article will cover the role of Snc1 in the anterograde and retrograde trafficking of fungi and its interactions with other proteins for efficient cellular transportation.

Keywords: SNARE complex; SNARE proteins; SNC1; protein secretion; protein trafficking; vesicle fusion.

Figure 1

Protein structure of Snc1 of Trichoderma reesei QM6a. (A) 3D structure of SNARE protein Snc1 (created at https://swissmodel.expasy.org (accessed on 7 April 2023)). (B) Schematic representation of Snc1 domains (created at https://smart.embl-heidelberg.de (accessed on 7 April 2023)). SNARE domain is represented in the orange color (residues 16–76) and the trans-membrane domain is represented in the dark-blue color (residues 87–109).

Figure 2

Trans-SNARE complex of the Snc1, Q-SNAREs, Rab-GTPase and SM proteins. The R-SNARE SNC1 forms a trans-SNARE complex with three Q-SNAREs (Sso1, Sso2 and Sec9). The Rab-GTPases and SM proteins (Sec1/Munc18) mediate the SNARE complex formation, as well as the docking and delivery of secretory vesicles to the plasma membrane in collaboration with SNARE proteins.

Figure 3

Role of SNARE protein SNC1 in vesicle trafficking and membrane fusion of fungi. SNC1 plays an essential role in anterograde and retrograde vesicle trafficking between the PM and Golgi. SNC1 interacts with exocytic SNAREs (SSo1, SSO2 and Sec9) and the exocytic complex (Sec3, Sec5, Sec6, Sec8, Sec15, Exo70, Exo84) for efficient exocytosis and releases cellular contents to the extracellular environment. The SM (Sec1/Munc18) proteins also play an important role during the exocytic fusion with the PM. During anterograde trafficking, SNC1 interacts with the endocytic proteins Tlg1, Tlg2, Sro7, Syp1, Ede1 and Pal1. The post-endocytic recycling of SNC1 is dependent upon Rcy1/COPI and Snx4 after endocytosis. The post-endocytic recycling of SNC1 adopts a minor route via the retromer. The Tlg1-positive early endosome first receives the endocytosed cargoes, which are then selectively transported by Rcy1/Drs2/COPI back to the Sec7-positive TGN or directly recycled back to the PM, or may remain in the early endosome as it matures into a PVE. Once the EE matures into a PVE, the specific cargoes are able to return by either the retromer or Snx4. The PVE will eventually fuse with the vacuole. Various other factors are also involved in the vesicular trafficking, such as Rab-GTPases, tethers and coat proteins.

Figure 4

Subcellular localization of Snc1 and related SNAREs of PM in Aspergillus oryzae. DIC and EGFP fluorescence micrographs of strains expressing the fusion proteins of (a) AoSso1p, (b) AoSso2p, (c) AoSnc1p, and (d) AoNyv1p. The bar represents 10 µm. (Reprint from “Systematic analysis of SNARE localization in the filamentous fungus Aspergillus oryzae”, by Kuratsu et al., [71], Copyright (2023), by Elsevier).

Figure 5

Subcellular localization of the endosome-related SNAREs of Aspergillus oryzae. DIC, EGFP, and FM4-64 or CMAC fluorescence micrographs of strains expressing the fusion proteins of (A) early endosome-resident SNAREs, including (a) AoTlg2p, (b) AoTlg1p, (c) AoVti1p, (d) AoSyn8p, and (e) AoSnc1p, and (B) late endosome-resident SNAREs, including (f) AoTlg2p, (g) AoVti1p, (h) AoNyv1p, (i) AoSyn8p, and (j) AoVam7p. Arrows in (A) indicate early endosomes that show directional movement along the hyphal axis, and arrows in (B) indicate late endosomes that are static and adjacent to vacuoles. The bar represents 10 µm (Reprint from “Systematic analysis of SNARE localization in the filamentous fungus Aspergillus oryzae”, by Kuratsu et al., [71], Copyright (2023), by Elsevier).