The carboxyl-terminal Src kinase (Csk) is an indispensable negative regulator for the Src family tyrosine kinases (SFKs) that play pivotal roles in various cell signalings. To understand the molecular basis of the Csk-mediated regulation of SFKs, we elucidated the crystal structure of full-length Csk. The Csk crystal consists of six molecules classified as active or inactive states according to the coordinations of catalytic residues. Csk assembles the SH2 and SH3 domains differently from inactive SFKs, and their binding pockets are oriented outward enabling the intermolecular interaction. In active molecules, the SH2-kinase and SH2-SH3 linkers are tightly stuck to the N-lobe of the kinase domain to stabilize the active conformation, and there is a direct linkage between the SH2 and the kinase domains. In inactive molecules, the SH2 domains are rotated destroying the linkage to the kinase domain. Cross-correlation matrices for the active molecules reveal that the SH2 domain and the N-lobe of the kinase domain move as a unit. These observations suggest that Csk can be regulated through coupling of the SH2 and kinase domains and that Csk provides a novel built-in activation mechanism for cytoplasmic tyrosine kinases.

The sterol regulatory element-binding protein 2 (SREBP-2), a nuclear transcription factor that is essential for cholesterol metabolism, enters the nucleus through a direct interaction of its helix-loop-helix leucine zipper domain with importin-beta. We show the crystal structure of importin-beta complexed with the active form of SREBP-2. Importin-beta uses characteristic long helices like a pair of chopsticks to interact with an SREBP-2 dimer. Importin-beta changes its conformation to reveal a pseudo-twofold symmetry on its surface structure so that it can accommodate a symmetric dimer molecule. Importin-beta may use a similar strategy to recognize other dimeric cargoes.

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