A novel autopolysialylation activity of the ganglioside sialyltransferase ST8Sia5 regulates its secretion and enzyme activity

Polysialic acid (polySia) is a linear polymer of sialic acid, which usually modifies N-glycans on the neural cell adhesion molecule (NCAM) mostly in the brain and is involved in the development of brain. PolySia is also associated with several diseases, including mental disorders and cancers. ST8Sia2 and ST8Sia4 are believed to be the only polysialyltransferases that synthesize polySia on NCAM (NCAM polysialylation). These enzymes are also autopolysialylated. In this study, we first found that ST8Sia5L, a ganglioside-specific sialyltransferase, has an activity to synthesize polysialic acid on ST8Sia5 itself, but does not exhibit the NCAM polysialylation activity. Notably, in silico and biochemical analyses revealed that ST8Sia5L contains a new polysialic acid trapping motif (PSTM) that is essential for polySia elongation, instead of the conventional polysialyltransferase domain (PSTD) found in ST8Sia2 and ST8Sia4. We also found that autopolysialylated ST8Sia5L is secreted from the cells. To identify the autopolysialylation sites involved in secretion, we performed N-glycosylation site disruption experiments, and found that N92 and N277 in the five N-glycosylation sites are important for this phenomenon. Furthermore, the inhibitor experiments showed that certain metalloprotease(s), but not exosomal pathways, are involved in the secretion. Notably, the secreted autopolysialylated enzyme showed no ganglioside-sialylation activity; however, the activity was recovered when polySia was removed by sialidase treatment. Overall, we show that autopolysialylation of ST8Sia5L regulates both its secretion and the conventional sialyltransferase activity.