The polysialyltransferase ST8Sia II/STX: posttranslational processing and role of autopolysialylation in the polysialylation of neural cell adhesion molecule
作者:B. E. Close、J. M. Wilkinson、T. J. Bohrer、C. P. Goodwin、L. J. Broom、K. J. Colley
DOI:10.1093/glycob/11.11.997
日期:2001.11.1
The presence of α2,8-linked polysialic acid on the neural cell adhesion molecule (NCAM) is known to modulate cell interactions during development and oncogenesis. Two enzymes, the α2,8-polysialyltransferases ST8Sia IV/PST and ST8Sia II/STX are responsible for the polysialylation of NCAM. We previously reported that both ST8Sia IV/PST and ST8Sia II/STX enzymes are themselves modified by α2,8-linked polysialic acid chains, a process called autopolysialylation. In the case of ST8Sia IV/PST, autopolysialylation is not required for enzymatic activity. However, whether the autopolysialylation of ST8Sia II/STX is required for its ability to polysialylate NCAM is unknown. To understand how autopolysialylation impacts ST8Sia II/STX enzymatic activity, we employed a mutagenesis approach. We found that ST8Sia II/STX is modified by six Asn-linked oligosaccharides and that polysialic acid is distributed among the oligosaccharides modifying Asn 89, 219, and 234. Coexpression of a nonautopolysialylated ST8Sia II/STX mutant with NCAM demonstrated that autopolysialylation is not required for ST8Sia II/STX polysialyltransferase activity. In addition, catalytically active, nonautopolysialylated ST8Sia II/STX does not polysialylate any endogenous COS-1 cell proteins, highlighting the protein specificity of polysialylation. Furthermore, immunoblot analysis of NCAM polysialylation by autopolysialylated and nonautopolysialylated ST8Sia II/STX suggests that the NCAM is polysialylated to a higher degree by autopolysialylated ST8Sia II/STX. Therefore, we conclude that autopolysialylation of ST8Sia II/STX, like that of ST8Sia IV/PST, is not required for, but does enhance, NCAM polysialylation.
已知神经细胞黏附分子(NCAM)上的α2,8连接的多唾酸可以调节发育和肿瘤发生过程中的细胞相互作用。两种酶,α2,8-多唾酸转移酶ST8Sia IV/PST和ST8Sia II/STX,负责对NCAM进行多唾酸化。我们之前报道过,ST8Sia IV/PST和ST8Sia II/STX酶本身也被α2,8连接的多唾酸链修饰,这一过程称为自我多唾酸化。对于ST8Sia IV/PST来说,自我多唾酸化对其酶活性并不是必需的。然而,ST8Sia II/STX的自我多唾酸化是否对其多唾酸化NCAM的能力是必需的尚不清楚。为了理解自我多唾酸化如何影响ST8Sia II/STX的酶活性,我们采用了诱变的方法。我们发现ST8Sia II/STX被六个Asn连接的寡糖修饰,且多唾酸分布在修饰Asn 89、219和234的寡糖中。非自我多唾酸化的ST8Sia II/STX突变体与NCAM的共表达表明,自我多唾酸化并不是ST8Sia II/STX多唾酸转移酶活性所必需的。此外,具有催化活性的非自我多唾酸化ST8Sia II/STX未能对任何内源性COS-1细胞蛋白进行多唾酸化,突显了多唾酸化的蛋白特异性。此外,通过自我多唾酸化和非自我多唾酸化的ST8Sia II/STX对NCAM多唾酸化的免疫印迹分析表明,NCAM在自我多唾酸化的ST8Sia II/STX的作用下多唾酸化的程度更高。因此,我们得出结论:ST8Sia II/STX的自我多唾酸化与ST8Sia IV/PST相似,并不是进行NCAM多唾酸化所必需的,但确实增强了NCAM的多唾酸化。