Protein O-mannose kinase that specifically mediates phosphorylation at the 6-position of an O-mannose of the trisaccharide (N-acetylgalactosamine (GalNAc)-beta-1,3-N-acetylglucosamine (GlcNAc)-beta-1,4-mannose) to generate phosphorylated O-mannosyl trisaccharide (N-acetylgalactosamine-beta-1,3-N-acetylglucosamine-beta-1,4-(phosphate-6-)mannose). Phosphorylated O-mannosyl trisaccharide is a carbohydrate structure present in alpha-dystroglycan (DAG1), which is required for binding laminin G-like domain-containing extracellular proteins with high affinity. Only shows kinase activity when the GalNAc-beta-3-GlcNAc-beta-terminus is linked to the 4-position of O-mannose, suggesting that this disaccharide serves as the substrate recognition motif (By similarity). 3-O-[beta-D-GalNAc-(1->3)-beta-D-GlcNAc-(1->4)-alpha-D-Man]-L-Thr-[protein] + ATP = 3-O-[beta-D-GalNAc-(1->3)-beta-D-GlcNAc-(1->4)-(O-6-P-alpha-D-Man)]-Thr-[protein] + ADP + H(+) Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. STKL subfamily. Although related to the Ser/Thr protein kinase family, has no protein kinase activity and acts as a mannose kinase instead. Extended N-terminus.