SAC1 degrades its lipid substrate PtdIns4 in the endoplasmic reticulum to maintain a steep chemical gradient with donor membranes.

Gradients of PtdIns4 between organelle membranes and the endoplasmic reticulum (ER) are thought to drive counter-transport of other lipids via non-vesicular traffic. This novel pathway requires the SAC1 phosphatase to degrade PtdIns4 in a 'cis' configuration at the ER to maintain the gradient. However, SAC1 has also been proposed to act in 'trans' at membrane contact sites, which could oppose lipid traffic.