Protein sialyltransferase specifically expressed in goblet cells that plays a key role in intestinal host-commensal homeostasis (PubMed:35303419). Conjugates sialic acid with an alpha-2-6 linkage to N-acetylgalactosamine (GalNAc) glycan chains linked to serine or threonine in glycoproteins (PubMed:10536037, PubMed:15466199, PubMed:16319059, PubMed:31719620, PubMed:35303419). Catalyzes the formation of the sialyl-Tn (S-Tn) antigen, an antigen found in intestinal goblet cells, as well as ulcerative colitis (UC) and various cancers (PubMed:15466199, PubMed:16319059, PubMed:31719620, PubMed:35303419). Protein sialylation in globlet cells is essential for mucus integrity and is required to protect the intestinal mucus against excessive bacterial proteolytic degradation (PubMed:35303419). {ECO:0000269|PubMed:10536037, ECO:0000269|PubMed:15466199, ECO:0000269|PubMed:16319059, ECO:0000269|PubMed:31719620, ECO:0000269|PubMed:35303419}. This is the function of ST6GALNAC1 (ST6 N-acetylgalactosaminide alpha-2,6-sialyltransferase 1, Ensembl gene identifier ENSG00000070526).