Rules and exceptions: sialic acid variants and their role in determining viral tropism.

Sialic acids decorate the surfaces of most mammalian cells and are used by many viruses as attachment receptors. In contrast to other mammals, humans cannot synthesize a version of sialic acid known as N-glycolyl neuraminic acid. This difference is exploited by some viruses to establish tropism. Here we compare recently determined structures of closely related animal and human polyomaviruses and examine their strategies for engaging specific sialic acid variants.