PDF(Pubmed)
Abstract：
Structural variation of N-glycans is essential for the regulation of glycoprotein functions. GalNAcβ1-4GlcNAc (LacdiNAc or LDN), a unique subterminal glycan structure synthesized by B4GALNT3 or B4GALNT4, is involved in the clearance of N-glycoproteins from the blood and maintenance of cell stemness. Such regulation of glycoprotein functions by LDN is largely different from that by the dominant subterminal structure, N-acetyllactosamine (Galβ1-4GlcNAc, LacNAc). However, the mechanisms by which B4GALNT activity is regulated and how LDN plays different roles from LacNAc remain unclear. Here, we found that B4GALNT3 and four have unique domain organization containing a noncatalytic PA14 domain, which is a putative glycan-binding module. A mutant lacking this domain dramatically decreases the activity toward various substrates, such as N-glycan, O-GalNAc glycan, and glycoproteins, indicating that this domain is essential for enzyme activity and forms part of the catalytic region. In addition, to clarify the mechanism underlying the functional differences between LDN and LacNAc, we examined the effects of LDN on the maturation of N-glycans, focusing on the related glycosyltransferases upstream and downstream of B4GALNT. We revealed that, unlike LacNAc synthesis, prior formation of bisecting GlcNAc in N-glycan almost completely inhibits LDN synthesis by B4GALNT3. Moreover, the presence of LDN negatively impacted the actions of many glycosyltransferases for terminal modifications, including sialylation, fucosylation, and human natural killer-1 synthesis. These findings demonstrate that LDN has significant impacts on N-glycan maturation in a completely different way from LacNAc, which could contribute to obtaining a comprehensive overview of the system regulating complex N-glycan biosynthesis.
摘要：
N-聚糖的结构变化对于糖蛋白功能的调节至关重要。GalNAcβ1-4GlcNAc（LacdiNAc或LDN），由B4GALNT3或B4GALNT4合成的独特的亚末端聚糖结构参与从血液中清除N-糖蛋白和维持细胞干细胞性。LDN对糖蛋白功能的这种调节与显性亚末端结构的调节有很大不同，N-乙酰乳糖胺（Galβ1-4GlcNAc，LacNAc）。然而,B4GALNT活性受调控的机制以及LDN如何与LacNAc发挥不同作用尚不清楚.这里,我们发现B4GALNT3和4具有包含非催化PA14结构域的独特结构域结构，这是一个推定的聚糖结合模块。缺乏该结构域的突变体会大大降低对各种底物的活性，如N-聚糖，O-GalNAc聚糖，和糖蛋白，表明该结构域对于酶活性是必需的并且形成催化区的一部分。此外,为了阐明LDN和LacNAc之间功能差异的潜在机制，我们研究了LDN对N-聚糖成熟的影响，重点关注B4GALNT上游和下游的相关糖基转移酶。我们透露，与LacNAc合成不同，在N-聚糖中形成二等分GlcNAc几乎完全抑制B4GALNT3的LDN合成。此外，LDN的存在对许多糖基转移酶末端修饰的作用产生负面影响，包括唾液酸化,岩藻糖基化,和人类自然杀手-1（HNK-1）合成。这些发现表明，LDN对N-聚糖成熟的影响与LacNAc完全不同，这可能有助于获得调节复杂N-聚糖生物合成的系统的全面概述。
