Regulatory component of the inositol phosphorylceramide (ICP) synthase which catalyzes the addition of a phosphorylinositol group onto ceramide to form inositol phosphorylceramide, an essential step in sphingolipid biosynthesis. Helps the medial Golgi localization of IPC synthase in a COPI vesicle-dependent manner (By similarity). Component of the inositol phosphorylceramide synthase complex composed of at least aur1 and kei1. Belongs to the KEI1 family.