Structure, attachment and entry of polyoma- and papillomaviruses.

Polyoma- (PY) and Papillomavirus (PV) virions have remarkable structural equivalence although no discernable sequence similarities among the capsid proteins can be detected. Their similarities include the overall surface organization, the presence of 72 capsomeres composed of five molecules of the major capsid proteins, VP1 and L1, respectively, the structure of the core segment of capsomeres with classical antiparallel "jelly roll" beta strands as the major feature, and the linkage of neighboring capsomeres by invading C-terminal arms. Differences include the size of surface exposed loops that contain the dominant neutralizing epitopes, the details of the intercapsomeric interactions, and the presence of 2 or 1 minor capsid proteins, respectively. These differences may affect the dramatic differences observed in receptor binding and internalization pathways utilized by these viruses, but as detailed later even structural differences cannot completely explain receptor and pathway usage. In recent years, technical advances aiding the study of entry processes have allowed the identification of novel endocytic compartments and an appreciation of the links between endocytic pathways that were previously thought to be completely separable. This review is intended to highlight recent advances in our understanding of virus receptor interactions and their consequences for endocytosis and intracellular trafficking.