Why multiple ligands for SR-A?
The most intriguing and defining property of SR-A is its unique pattern of ligand-binding activities. Together with its expression on the ubiquitous and highly versatile Mφ, the diversity of ligands would seem to predict a multitude of functions for this molecule (35). Conversely, the reason Mφ can perform multiple tasks is in part because of their expression of molecules such as SR-A. We might wonder whether this strategy of employing a single receptor to fulfil several requirements is advantageous compared with the situation of having many receptors, each performing more restricted functions. It is not obvious that there is a disadvantage to the latter, because the vast majority of plasma membrane receptors are very narrow in the range of molecules with which they can interact. Typically they have a limited number of ligands. There does not seem to be any sort of theoretical limit on the generation of receptor diversity. Even when inherited material becomes limiting, genetic recombin
