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We had previously identified the mutant allele of  apm1 +   that encodes a homolog of the mammalian μ 1A subunit of the clathrin-associated adaptor protein-1 (AP-1) complex and demonstrated that the AP-1 complex plays a role in Golgi/endosome trafficking, secretion, and vacuole fusion in fission yeast. Here, we isolated a mutant allele of  its4 + /sip1 +  , which encodes a conserved AP-1 accessory protein. The  its4-1 / sip1-i4  mutants and  apm1   - deletion cells exhibited similar phenotypes, including sensitivity to the calcineurin inhibitor FK506, Cl −  and valproic acid as well as various defects in Golgi/endosomal trafficking and cytokinesis. Electron micrographs of  sip1-i4  mutants revealed vacuole fragmentation and accumulation of abnormal Golgi-like structures and secretory vesicles. Overexpression of Apm1 suppressed defective membrane trafficking in  sip1-i4  mutants. The Sip1-green fluorescent protein (GFP) co-localized with Apm1-mCherry at Golgi/endosomes, and Sip1 physically interacted with each subunit of the AP-1 complex. We found that Sip1 was a Golgi/endosomal protein and the  sip1-i4  mutation affected AP-1 localization at Golgi/endosomes, thus indicating that Sip1 recruited the AP-1 complex to endosomal membranes by physically interacting with each subunit of this complex. Furthermore, Sip1 is required for the correct localization of Bgs1/Cps1, 1,3-β-D-glucan synthase to polarized growth sites. Consistently, the  sip1-i4  mutants displayed a severe sensitivity to micafungin, a potent inhibitor of 1,3-β-D-glucan synthase. Taken together, our findings reveal a role for Sip1 in the regulation of Golgi/endosome trafficking in coordination with the AP-1 complex, and identified Bgs1, required for cell wall synthesis, as the new cargo of AP-1-dependent trafficking.

Sip1, a Conserved AP-1 Accessory Protein, Is Important for Golgi/Endosome Trafficking in Fission Yeast