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. Component of the inositol phosphorylceramide synthase complex composed of at least AUR1 and KEI1. Interacts (via C-terminal region) with COP1 and SEC21. Note=The interaction with AUR1 seems to occur with the full-length protein before cleavage by KEX2 since both full-length and short chains of KEI1 interact with AUR1. The precursor protein is cleaved into two polypeptide chains, KEI1N and KEI1C. The cleavage is performed in the Golgi apparatus by the KEX2 protease which recognizes residue Arg-135. Generation of KEX2 cleavage site may have been an accidental event in evolution without specific advantages or disadvantages in IPC synthesis. Present with 3420 molecules/cell in log phase SD medium. Belongs to the KEI1 family.