Identification of an <scp>ERGIC</scp> ‐Like Compartment in Fission Yeast: Emp43 Functions as a Lectin‐Like Cargo Receptor for Glycosylated Proteins

The endoplasmic reticulum-Golgi intermediate compartment (ERGIC) plays a crucial role in the secretory pathway; however, its existence and function in lower eukaryotes remain largely unexamined. In this study, we identified Emp43 (SPBC4F6.05c) of Schizosaccharomyces pombe, an orthologue of human (Homo sapiens) ERGIC-53, and demonstrated its localization to an ERGIC-like compartment. The localization of Emp43 depended on its C-terminal KYL motif and oligomerization through the CC1 domain. Deletion of S. pombe emp43⁺ resulted in significant sensitivity to MgCl₂ and FK506, along with defects in septum integrity, indicating a role in cell wall maintenance. Further analysis identified Ssp120 of S. pombe, an orthologue of human MCFD2, as a functional partner of Emp43. Yeast two-hybrid assays confirmed a strong interaction between Emp43 and Ssp120, and both proteins co-localized within an ERGIC-like compartment. Additionally, we identified Meu17 of S. pombe, a glucan-α-1,4-glucosidase homolog, as a potential ligand for Emp43. Overexpression of Meu17 rescued MgCl₂ sensitivity in both emp43Δ and ssp120Δ strains, while mutations in its N-linked glycosylation sites (N383, N409) or its predicted active site (D203) disrupted its septum localization and functional rescue capability. Our findings indicate that Emp43 forms a complex with Ssp120 to facilitate the transport of glycosylated proteins, such as Meu17, within an ERGIC-like compartment in fission yeast S. pombe. This study provides the first evidence of an ERGIC-like structure in S. pombe and highlights the conserved nature of ERGIC-associated mechanisms across eukaryotes.