Loss of Apm1, the mu 1 Subunit of the Clathrin-Associated Adaptor-Protein-1 Complex, Causes Distinct Phenotypes and Synthetic Lethality with Calcineurin Deletion in Fission Yeast

Calcineurin is a highly conserved regulator of Ca super(2+) signaling in eukaryotes. In fission yeast, calcineurin is not essential for viability but is required for cytokinesis and Cl super(-) homeostasis. In a genetic screen for mutations that are synthetically lethal with calcineurin deletion, we isolated a mutant, cis1-1/apm1-1, an allele of the apm1 super(+) gene that encodes a homolog of the mammalian mu 1A subunit of the clathrin-associated adaptor protein-1 (AP-1) complex. The cis1-1/apm1-1 mutant as well as the apm1- deleted ( Delta apm1) cells showed distinct phenotypes: temperature sensitivity; tacrolimus (FK506) sensitivity; and pleiotropic defects in cytokinesis, cell integrity, and vacuole fusion. Electron micrographs revealed that Delta apm1 cells showed large vesicular structures associated with Golgi stacks and accumulated post-Golgi secretory vesicles. Delta apm1 cells also showed the massive accumulation of the exocytic v-SNARE Syb1 in the Golgi/endosomes and a reduced secretion of acid phosphatase. These phenotypes observed in apm1 mutations were accentuated upon temperature up-shift and FK506 treatment. Notably, Apm1-GFP localized to the Golgi/endosomes, the spindle pole bodies, and the medial region. These findings suggest a role for Apm1 associated with the Golgi/endosome function, thereby affecting various cellular processes, including secretion, cytokinesis, vacuole fusion, and cell integrity and also suggest that calcineurin is involved in these events.