1 Introduction

Abstract

I know death has ten thousand several doors…

John Webster

ca. 1610

To escape their existence as mere chemicals, viruses must find cellular receptors to invade a host cell and, thus, enter the life cycle of reproduction. Invasion of the host cell by viruses leads, in most cases, to destruction of the cell, even to the death of the host organism. The receptor, the first cellular molecule encountered by the virus, is an all-essential component of the cellular gate that, unbeknown to the cell, will allow entry of a deadly agent.

Whereas finding the host cell is a chance encounter, the steps of docking onto the cellular receptor, penetration, and uncoating follow elaborate viral strategies. A most surprising discovery in studies of these early events of infection has been the diversity of cellular receptors that viruses have selected for entry. It seems as if any cell-surface molecule, regardless of its structure and cellular function, can facilitate attachment for a specific viral species. To be sure, attachment per se does not necessarily lead to uptake and productive infection, but most viruses are clever enough to have chosen only receptors that are expressed on cells suitable for uptake and replication.

Several years ago, it seemed plausible that certain classes of abundant cell-surface molecules, such as proteins belonging to the immunoglobulin superfamily, might provide the preferred menu from which a virus is likely to choose its receptor (White and Littman 1989). However, a deluge of new data has shown that this is not...