Current ideas on the significance of protein glycosylation

Carbohydrate has been removed from a number of glycoproteins without major effect on the structure or enzyme activity of the protein. Thus carbohydrate has been suggested to underly a non-primary function for proteins, such as in relatively non-specific interactions with other carbohydrates or macromolecules, stabilization of protein conformation, or protection from proteolysis. This non-specific concept is consistent with both the general similarity in carbohydrate structure on very diverse glycoproteins and the frequent structural microheterogeneity of carbohydrate chains at given sites. The concept is supported in a general sense by the viability of cells whose glycosylation processes have been globally disrupted by mutation or pharmacological inhibitors. In contrast to the above observations, other studies have revealed the existence of specific, selective receptors for discrete oligosaccharide structures on glycoproteins which seem to be important for compartmentalization of the glycoprotein, or the positioning of cells on which the glycoprotein is concentrated. Sometimes multivalency in the carbohydrate-receptor interaction is crucial. There are additional possible roles for carbohydrate in the transduction of information upon binding to a receptor. The possibility of specific roles for carbohydrate is supported by the existence of numerous unique carbohydrate structures, many of which have been detected as glycoantigens by monoclonal antibodies, with unique distributions in developing and differentiated cells. This article attempts to summarize and rationalize the contradictory results. It appears that in general carbohydrate does in fact underlie only roles secondary to a protein's primary function. These secondary roles are simple non-specific ones of protection and stabilization, but often also satisfy the more sophisticated needs of spatial position control and compartmentalization in multicellular eukaryotic organisms. It is suggested that there are advantages, evolutionarily speaking, for the shared use of carbohydrate for non-specific roles and for specific roles primarily as luxury functions to be executed during the processes of cell differentiation and morphogenesis.