Conserved proteins are fragile

Levels of selective constraint vary among proteins. Although strong constraint on a protein is often attributed to its functional importance, evolutionary rate may also be limited if a protein is fragile, such that a large proportion of amino acid replacements reduce its fitness. To determine the relative contributions of essentiality and fragility to selective constraint, we compared relationships of selection against nonsense mutations (s _non) and selection against missense mutations (s_mis) to protein sequence conservation (K_a). As expected, s_non is greater than s_mis; however, the correlation between s_mis and K_a is nearly three times stronger than the correlation between s_non and K_a. Moreover, examination of relationships to gene expression level, tissue specificity, and number of protein-protein interactions shows that s_mis is more strongly correlated than s _non to all three measures of biological function. Thus, our analysis reveals that slowly evolving proteins are under strong selective constraint primarily because they are fragile, and that this association likely exists because allowing a protein to function improperly, rather than removing it from a biological network, can negatively affect the functions of other molecules it interacts with and their downstream products.