Mucus glycoproteins, super glycoforms: how to solve a sticky problem?

Targeting MUC15 Protein in Cancer: Molecular Mechanisms and Therapeutic Perspectives

MUC15, a member of the mucin family, is a heavily glycosylated transmembrane protein with the primary functions of lubricating surfaces, establishing a selective molecular barrier at the epithelium and mediating signal transduction. Aberrant expression of MUC15 plays a crucial role in the progression of multiple diseases, including malignant tumors. MUC15 has been identified as a tumor suppressor, but current evidence indicate its function as an oncogene in different types of cancers. MUC15 has been shown to be involved in the development of cancer and influence cellular growth, adhesion, invasion, metastasis and immune immunomodulation. However, the precise role of MUC15 in tumour development has not been thoroughly clarified. Here, we systematically summarize the structure and function of MUC15 in cancer, and discuss its potential role in cancer treatment.

Bovine submaxillary mucin contains multiple domains and tandemly repeated non-identical sequences

A number of cDNA fragments coding for bovine submaxillary mucin (BSM) were cloned, and the nucleotide sequence of the largest clone, BSM421, was determined. Two peptide sequences determined from the purified apoBSM were found near the N-terminus of the mucin-coding region of BSM421. This clone does not contain a start or stop codon, but its 3' end overlaps with the 5' end of a previously isolated clone, lambdaBSM10. The composite sequence of 1589 amino acid residues consists of five distinct protein domains, which are numbered from the C-terminus. The cysteine-rich domain I can be further divided into a von Willebrand factor type C repeat and a cystine knot. Domains III and V consist of similar repeated peptide sequences with an average of 47 residues. Domains II and IV do not contain such sequences but are similar to domains III and V in being rich in serine and threonine, many of which are predicted to be potential O-glycosylation sites. Domain III also contains two sequences that match the ATP/GTP-binding site motif A (P-loop). Only beta-strands and no alpha-helices are predicted for the partial deduced amino acid sequence. Northern analysis of submaxillary gland RNA with the BSM421 probe detected multiple messages of BSM with sizes from 1.1 to over 10 kb. The tandemly repeated, non-identical peptide sequences of approx. 47 residues in domains III and V of BSM differ from the tandemly repeated, identical 81-residue sequences of pig submaxillary mucin (PSM), although both BSM and PSM contain similar C-terminal domains. In contrast, two peptide sequences of ovine submaxillary mucin are highly similar (86% and 65% identical respectively) to the corresponding sequences in domain V of BSM.

Cell-cell interactions influence oligosaccharide modifications on mucins and other large glycoproteins

Intratumoral phenotypic diversity is well documented with regard to tumor associated carbohydrate antigens (TACA). The factors which control the expression of these cell-surface oligosaccharides on different cells of the same tumor are not understood. We investigated the expression of a panel of mucin associated oligosaccharides in cell lines growing at different surface densities (number of cells per cm2 of growth flask). Results show that the apparent expression of extended Lea-Lex, Lea and Lex, sialyl Lea, Tn and sialyl Tn varies with density of growth by an invasive human squamous cell lung carcinoma cell line (NU6-1), a benign variant (NE-18) and the human lung epithelial cell line BEAS-2B. The results indicate that one of the factors influencing the apparent expression of mucin-associated oligosaccharides is cell-cell interactions.

Purification and structural characterization of a filamentous, mucin-like proteophosphoglycan secreted by Leishmania parasites

Parasitic protozoa of the genus Leishmania secrete a filamentous macromolecule that forms networks and appears to be associated with cell aggregation. We report here the purification of this parasite antigen from Leishmania major culture supernatant and its compositional (75.6% carbohydrate, 20% phosphate, 4.4% amino acids, w/w), structural, and ultrastructural characterization as a highly unusual proteophosphoglycan (PPG). Mild acid hydrolysis, which cleaves preferentially hexose 1-phosphate bonds, releases the PPG glycans. Their structures are Galbeta1-4Man, Manalpha1-2Man, Galbeta1-3Galbeta1-4Man, PO4-6(Galbeta1-3)0-2Galbeta1-4Man, and PO4-6(Arabeta1-2Galbeta1-3)Galbeta1-4Man. These glycans are also components of the parasite glycolipid lipophosphoglycan, but their relative abundance and structural organization in PPG are different. Some of them represent novel forms of protein glycosylation. 31P NMR on native PPG demonstrates that phosphate is exclusively in phosphodiester bonds and that the basic structure R-Manalpha1-PO4-6-Gal-R connects the glycans. A phosphodiester linkage to phosphoserine (most likely R-Manalpha1-PO4-Ser) anchors the PPG oligosaccharides to the polypeptide. PPG has a unique amino acid composition; glycosylated phosphoserine (>43 mol %), serine, alanine, and proline account for more than 87 mol % and appear to be clustered in large proteinase-resistant domains. Electron microscopy of purified PPG reveals cable-like, flexible, long (to 6 microm), and unbranched filaments. The overall structure of PPG shows many similarities to mammalian mucins. Potential functions of this novel mucin-like molecule for the parasites are discussed.