Researchers follow a plasma membrane glycoprotein as it matures. Its early form carries oligosaccharides rich in mannose, while the finished molecule ends in negatively charged sialic acid residues. Where do those terminal sugars get added?

Core Concept

Glycosylation follows an ordered spatial sequence across the Golgi: early cisternae trim mannose, middle cisternae add GlcNAc and galactose, and transferases concentrated nearest the trans end cap the chains with sialic acid. Locating terminal sugars therefore places the finishing step at the stack's far side. Each cisterna's distinct enzyme complement enforces this order.