Gifu University discovers enzyme that regulates glycan elongation: Regulating the lifespan of blood proteins

A research group led by Professor Yasuhiko Kizuka of the Institute for Glyco-core Research at Gifu University, in collaboration with the University of Lille in France, Osaka University and Hiroshima University, discovered a new mechanism to stop the elongation of glycans attached to proteins. The findings are expected to contribute to elucidation of the bone formation mechanism and modification of the blood protein stability using these molecules. The results were published in the Journal of Biological Chemistry.

B4GALNT3, a type of human glycosyltransferase, acts on N- and mucin-type glycans and elongates glycans attached to proteins. The glycans made by B4GALNT3 have a structure called LDN (LacdiNAc), but many other N-glycans have a different structure called LacNAc, instead of LDN. Usually, when a glycan is elongated and LacNAc is attached, the glycan is further elongated, and sialic acid is attached to the terminal.

In B4GALNT3-deficient mice, the level of the glycoprotein sclerostin in the blood is markedly increased, resulting in abnormal bone formation. However, it was not known why the loss of LDN leads to increased sclerostin levels in the blood. Kizuka and his colleagues hypothesized that the attachment of LDN would negatively affect the glycan elongation. Conversely, when B4GALNT3 is deleted, glycans with LacNAc instead of LDN increase and are further elongated and sialylated to increase the stability of the proteins in the blood.

First, cultured cells were transfected with B4GALNT3, and cell surface glycans with sialic acid were detected. The amount of sialic acid in transfected cultured cells was decreased compared with that in cultured cells without transfection. To confirm whether sialic acid is actually attached to the glycans with LDN, they prepared LacNAc-containing and LDN-containing glycans and mixed them with an enzyme that adds sialic acid to the terminal of glycans (ST3GAL4) to see if sialic acid was attached to the glycans. As a result, sialic acid was attached to the terminal of the LacNAc-containing glycan, whereas almost no sialic acid was attached to the terminal of the LDN-containing glycan. In other words, the attachment of LDN to the glycan prevents the attachment of sialic acid to, and further elongation of, the glycan. This indicates that B4GALNT3 is an important glycosyltransferase controlling the lifespan of blood proteins by regulating glycan elongation.

Close examination of N-glycan termini revealed that multiple structures present at the terminals are difficult to attach to LDN-containing glycans, as is the case with sialic acid. Therefore, the B4GALNT3 action to produce LDN prevents the attachment of sialic acid and the production of important terminal structures of N-glycans, indicating that B4GALNT3 is an important regulator of N-glycan structure and function.

Journal Information
Publication: Journal of Biological Chemistry
Title: LacdiNAc synthase B4GALNT3 has a unique PA14 domain and suppresses N-glycan capping
DOI: 10.1016/j.jbc.2024.107450