Domain coverage by protein architecture type.

<p>Coverage of <i>all</i> domains by protein architecture type (top). “onlyrare” refer to proteins comprising one or more rare domains. “onlypromis” refer to proteins comprising one or more promiscuous domains. “mixed” refer to proteins comprising rare and promiscuous domains. Coverage of the 642 <i>gold</i> domains by protein architecture types (bottom). A gold domain is a domain that participates in at least one gold standard domain-domain interaction (GDDI). For rare domain threshold values between 2 and 4 inclusive, even though only-rare proteins involve a larger fraction of all domains, they cover a smaller fraction of gold domains than only-promiscuous proteins. Thus gold domains are more likely to be promiscuous domains. Mixed architecture proteins provide the largest coverage of gold domains when the rare domain threshold is between 3 and 5 inclusive. This range lies within the range where mixed architecture proteins make up the largest proportion of the protein population (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0088943#pone-0088943-g004" target="_blank">Figure 4</a>). So there exists a sweet spot where mixed architecture proteins are the most popular protein type and provide the largest coverage of gold domains.