Why mammalian cell surface proteins are glycoproteins

Most proteins presented at the external surface of mammalian cells contain carbohydrate. The reason for this is not fully understood, but recent work has shown that such carbohydrate has two major functions. Inside the cell, it helps proteins fold and assemble correctly in the endoplasmic reticulum, and it might also act as a signal for the correct migration of glycoproteins. Outside the cell, it provides specific recognition structures for interaction with a variety of external ligands.