Acts as a co-chaperone with HSPA8/Hsc70; required to promote protein folding and trafficking, prevent aggregation of client proteins, and promote unfolded proteins to endoplasmic reticulum-associated degradation (ERAD) pathway. Acts by determining HSPA8/Hsc70's ATPase and polypeptide-binding activities. Can also act independently of HSPA8/Hsc70: together with DNAJB14, acts as a chaperone that promotes maturation of potassium channels KCND2 and KCNH2 by stabilizing nascent channel subunits and assembling them into tetramers. While stabilization of nascent channel proteins is dependent on HSPA8/Hsc70, the process of oligomerization of channel subunits is independent of HSPA8/Hsc70. When overexpressed, forms membranous structures together with DNAJB14 and HSPA8/Hsc70 within the nucleus; the role of these structures, named DJANGOs, is still unclear. Homodimer and homotetramer. Interacts (via J domain) with HSPA8/Hsc70. Forms a multiprotein complex, at least composed of DNAJB12, DNAJB14, HSPA8/Hsc70 and SGTA; interaction with DNAJB14 and HSPA8/Hsc70 is direct. Localizes to the endoplasmic reticulum membrane. When overexpressed, forms membranous structures in the nucleus. Methylated at His-185 by METTL9. Belongs to the DnaJ family. DNAJB12/DNAJB14 subfamily.