The Unique Domain Forms a Fuzzy Intramolecular Complex in Src Family Kinases

The N-terminal regulatory region of c-Src including the SH4, Unique, and SH3 domains adopts a compact, yet highly dynamic, structure that can be described as an intramolecular fuzzy complex. Most of the long-range interactions within the Unique domain are also observed in constructs lacking the structured SH3, indicating a considerable degree of preorganization of the disordered Unique domain. Here we report that members of the Src family of kinases (SFK) share well-conserved sequence features involving aromatic residues in their Unique domains. This observation contrasts with the supposed lack of sequence homology implied by the name of these domains and suggests that the other members of SFK also have a regulatory region involving their Unique domains. We argue that the Unique domain of each SFK is sensitive to specific input signals, encoded by each specific sequence, but the entire family shares a common mechanism for connecting the disordered and structured domains.