Protein N-lysine methyltransferase that specifically trimethylates 'Lys-315' of VCP/p97; this modification may decrease VCP ATPase activity. L-lysyl-[protein] + 3 S-adenosyl-L-methionine = 3 H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl-[protein] + 3 S-adenosyl-L-homocysteine Interacts with ALKBH6. Interacts with ASPSCR1 and UBXN6; interaction with ASPSCR1, but not with UBXN6, enhances VCP methylation. Widely expressed. Deficient mice are viable, fertile and have no obvious pathological phenotype. Belongs to the methyltransferase superfamily. METTL21 family.