Fungal a-mannosidase Ams1 and its mammalian homolog MAN2C1 hydrolyze terminal a-linked mannoses in free oligosaccharides released from misfolded glycoproteins or lipid-linked oligosaccharide donors. Ams1 is transported by selective autophagy into vacuoles. Here, we determine the tetrameric structure of Ams1 from the fission yeast Schizosaccharomyces pombe at 3.2 A resolution by cryo-electron microscopy. Distinct from a low resolution structure of S. cerevisiae Ams1, S. pombe Ams1 has a prominent N-terminal tail that mediates tetramerization and an extra bsheet domain. Ams1 shares a conserved active site with other enzymes in glycoside hydrolase family 38, to which Ams1 belongs, but contains extra N-terminal domains involved in tetramerization. The atomic structure of Ams1 reported here will aid understanding of its enzymatic activity and transport mechanism. Asparagine-linked glycans (N-glycans) are one of the most important modifications in eukaryotic proteins and plays a crucial role in protein folding and quality control and molecular recognition [1,2]. Biosynthesis of N-glycans starts with the assembly of a donor substrate dolicholpyrophosphate-linked Glc 3 Man 9 GlcNAc 2 (Glc: glucose; Man: mannose; GlcNAc: N-acetylglucosamine) on the membrane of the endoplasmic reticulum (ER). The lipid-linked oligosaccharide is transferred to selected asparagine residues in nascent polypeptides by the oligosaccharyltransferase. N-glycans are then trimmed by glycosidases and extended by glycosyltransferases in the ER and Golgi, giving rise to their myriad structures. Catabolism of glycoproteins is important for maintaining cellular homeostasis. Most N-glycoproteins are degraded in the lysosome/vacuole [3], where N-glycans are digested by exoglycosidases at the nonreducing end and released from peptides with cleavage of the glycan-peptide bond at the reducing end. In addition, nascent glycoproteins that do not correctly fold in the ER are targeted for ER-associated degradation, a process in which misfolded glycoproteins are recognized