Synaptojanin family members are implicated in endocytic membrane traffic in yeast

The synaptojanins represent a subfamily of inositol 5'-phosphatases that co ntain an NH2-terminal Sac1p homology. domain. A nerve terminal-enriched syn aptojanin, synaptojanin 1, was previously proposed to participate in the en docytosis of synaptic vesicles and actin function. The genome of Saccharomy ces cerevisiae contains three synaptojanin-like genes (SJL1, SJL2 and SJL3) , none of which is essential for growth. We report here that a yeast mutant lacking SJL1 and SJL2 (Delta sjl1 Delta sjl2) exhibits a severe defect in receptor-mediated and fluid-phase endocytosis. A less severe endocytic defe ct is present in a Delta sj2 Delta sjl3 mutant, while endocytosis is normal in a Delta sjl1 Delta sjl3 mutant. None of the mutants are impaired in inv ertase secretion, The severity of the endocytic impairment of the sjl doubl e mutants correlates with the severity of actin and polarity defects. Furth ermore, the deletion of SJL1 suppresses the temperature-sensitive growth de fect of sac6, a mutant in yeast fimbrin, supporting a role for synaptojanin family members in actin function. These findings provide a first direct ev idence for a role of synaptojanin family members in endocytosis and provide further evidence for a close link between endocytosis and actin function.