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COVER FIGURE
Topology diagrams of factor XI dimer interfaces. Factor XI is a dimer with 2 identical subunits that form an interface through their A4 domains. Shown are the interfaces from the crystal structure for human factor XI (one A4 domain in yellow and the other in white) and a predicted structure for duck-billed platypus factor XI (one A4 domain in brown and the other in white). Key residues that form the interface are indicated in green. In factor XI from placental mammals, an interchain disulfide bond (Cys³²¹-Cys³²¹) covalently links the 2 subunits of the dimer. This is a relatively modern feature not found in factor XI from more primitive mammals, including most marsupials and egg-laying monotremes such as the platypus, where an Asp³²¹-Arg³²⁵ salt bridge contributes to the noncovalent interactions between the A4 domains. See the article by Ponczek et al.