Those sugary glycan moieties that adorn cell surface receptors are more than decoration. Their ability to prevent receptor endocytosis is well established. Now, Ken Lau, James Dennis (University of Toronto, Canada), and colleagues show that differences in receptors' glycan decorations time a cell's transition from growth to arrest.
Receptors that promote growth generally have more sites for glycan addition than do receptors that halt growth and start differentiation. The authors found that these receptor groups responded differently to changes in metabolite status, which determines the complexity of the added glycans (more sugar-nucleotides means more intricately branched glycans are created in the Golgi).
With their many glycans, growth receptors were cross-linked by sugar-binding galectins and retained on the surface even in stringent growth conditions. Receptors that promote differentiation required higher sugar-nucleotide levels before their fewer glycans gained enough galectin-binding branches to counter their loss by endocytosis.
The upshot, says Dennis, is “a principle of how cells regulate the ratio of growth and arrest receptors in a cell-autonomous manner downstream of nutrients. First, an increase in proliferation is accompanied by glucose uptake and increased metabolism.” Then when metabolite flux sufficiently increases sugar-nucleotides and the branched glycans, differentiation receptors can accumulate, turning off proliferation.