Molecule of the Month: Oligosaccharyltransferase

Almost all living cells are covered with a complex coating of carbohydrates. These carbohydrates are connected to the proteins and lipids that make up cell membranes. They are built of several types of simple sugars connected together in myriad different ways. This carbohydrate coating plays many essential roles in the life of the cell. Some are structural: for example, carbohydrates are bulky and form a protective shield that controls access to the cell surface. Secreted proteins, such as blood serum proteins, are often glycosylated, which improves their solubility and stability. Carbohydrates are also built with multiple shapes and sizes, and different organisms typically have their own distinctive collection of characteristic carbohydrates. The ABO blood types are a familiar example of personal variations in our own cell surface glycosylation, and in medical treatments such as blood transfusions, we have to avoid use of donors that are different than our own pattern of glycosylation.

Most commonly, carbohydrates are attached to proteins in two ways: at the oxygen atom in serine or threonine (O-glycosylation), or to the nitrogen in asparagine (N-glycosylation). Oligosaccharyltransferases perform the N-glycosylation reaction. Our cells make two similar forms of this enzyme: OST-A (shown here from PDB entry 6s7o) and OST-B (PDB entry 6s7t, not shown). First, a collection of diverse enzymes construct the carbohydrate sugar-by-sugar on a lipid carrier. Then oligosaccharyltransferase, as the name implies, transfers the carbohydrate from the lipid to an asparagine amino acid on the protein. In our cells, this transfer occurs in the endoplasmic reticulum, and then the carbohydrate is further modified and trimmed by additional enzymes in the endoplasmic reticulum and Golgi before being transported to the cell surface or exported out of the cell.

We have a constant tug of war with viruses, fought around glycosylated proteins. Viruses need to find cells and infect them, and cell surface carbohydrates are one way that cells physically protect themselves from viruses. However, viruses have evolved to evade this protection. For example, influenza viruses use hemagglutinin molecules to recognize carbohydrates on cell surfaces and use them during infection. Our immune system is also caught up in this battle. The intrinsic immune system recognizes unfamiliar carbohydrates on pathogen surfaces, such as the unusual glycosylated lipids on bacterial surfaces. But life-threatening viruses like HIV and SARS-CoV-2 coat themselves with human-like carbohydrates to help them evade our immune system.

Oligosaccharyltransferase-A works with the protein synthesis machinery to add carbohydrates to proteins as they are built. The structure shown here (PDB entry 6fti) shows one step of the process, after the transfer reaction has been performed. A ribosome is building a protein chain, which is then transported into the endoplasmic reticulum by the protein-conducting channel Sec61. Oligosaccharyltransferase adds carbohydrates at appropriate asparagines in the new protein chain. It recognizes a distinctive signal in the protein sequence, adding carbohydrates at asparagines that don't have proline next in the chain, and have either serine or threonine two amino acids later in the chain.