The function of METTL21A (methyltransferase 21A, HSPA lysine, ENSG00000144401) is as follows. Protein-lysine methyltransferase that selectively trimethylates residues in heat shock protein 70 (HSP70) family members. Contributes to the in vivo trimethylation of Lys residues in HSPA1 and HSPA8. In vitro methylates 'Lys-561' in HSPA1, 'Lys-564' in HSPA2, 'Lys-585' in HSPA5, 'Lys-563' in HSPA6 and 'Lys-561' in HSPA8. {ECO:0000269|PubMed:22948820, ECO:0000269|PubMed:23349634, ECO:0000269|PubMed:23921388}.