Crystal structures of Fsc1, a novel autophagy factor that mediates autophagosome-vacuole fusion in fission yeast.

Fsc1 is a recently identified autophagy factor in the fission yeast Schizosaccharomyces pombe that is implicated in the autophagosome-vacuole fusion step during the final stages of autophagy. Despite its critical role, the structural basis of Fsc1 function has remained unknown. Here, we report the first crystal structures of the luminal domain of Fsc1, revealing an elongated, modular architecture composed of five tandem fasciclin (FAS1) domains. Each domain adopts a hallmark β-sandwich fold, and the overall assembly forms a continuous scaffold featuring a conserved surface groove within the FAS1-4 domain. Structural and biochemical analyses demonstrate that Fsc1 forms a homodimer in solution through a shared interface observed in two independent crystal forms, supporting a biologically relevant but potentially low-affinity association. Comparative sequence and structural analyses reveal significant homology between Fsc1 and human fasciclin proteins, including TGFBI and periostin, suggesting similar structural principles underlying their functions. Together, these findings provide the first structural insights into Fsc1 and establish a structural framework for understanding how its modular architecture and context-dependent dimerization may facilitate late-stage membrane fusion during autophagy.