Ser/Thr 33 T-antigen Ser/Thr 3GalT Tn-antigen 6GlcNAcT 33 66 Ser/Thr Core 2 33 33 ST3Gal-I, II, IV 33 66 disialyl T-antigen ST6GalNAc III, IV, I, II sialyl Tn- antigen 66 ST6GalNAc-I 33 66 sialyl T-antigen ST6GalNAc-II, I
Cosmc Tn-antigens accumulate due to loss of 3Gal transferase activity No mutations in 3GalT; message is expressed Missing Cosmc (Core 1 3GalT -specific molecular chaperone, Xq23 25% identity, >40% homology to 3GalT Absence of Cosmc results in proteosome degradation of 3GalT Cosmc Ju & Cummings (2002) PNAS 99: Ser/Thr Tn-antigen Ser/Thr 33 T-antigen 3GalT
His-tagged Cosmc HPC4 tagged T- synthase Ni+ purification, Western with mAb to HPC4 T-synthase T-synthase + Mut Cosmc T-synthase + Wt Cosmc Total Not bound to Ni+ Bound to Ni+ Is Cosmc a Chaperone?
Outer Chain Assembly Sequential action of 4GalT and 3GlcNAcT gives rise to polylactosamine chains (Type II repeats) Type I repeats (Gal 3GlcNAc 3) also occur GlcNAc 6Gal branches (I-antigen) can occur The ends of the chains are capped in - linked sugars, e.g. 3/4Fuc and 3/6sialic acids Terminal structures make up important blood group determinants, e.g. the Lewis antigens 44 33 44 33 44 33 33 33 Ser/Thr 33 66
Intestinal Epithelium stained with Periodic acid-Schiff’s Reagent
Mucins 11p15 family (MUC2, MUC5AC, MUC5B, MUC6) probably responsible for the formation of mucus layers 7q22 family (MUC3A, MUC3B, MUC12), 1q21 (MUC1), and 3q (MUC4, MUC13) are membrane mucins Mucin families are not necessairly related to each other, convergent evolution Dekker et al. (2002) TIBS 27:126