Abstract
Proline switches, controlled by
cis–trans
isomerization, have emerged as a particularly effective regulatory mechanism in a wide range of biological processes. Here we report the structures of both the
cis
and
trans
conformers of a proline switch in Crk signaling protein. Proline isomerization toggles Crk between two conformations: an autoinhibitory, stabilized by the intramolecular association of two tandem SH3 domains in the
cis
form, and an uninhibited, activated conformation promoted by the
trans
form. In addition to acting as a structural switch the heterogeneous proline recruits cyclophilin A, which accelerates the interconversion rate between the isomers thereby regulating the kinetics of Crk activation. The data provide atomic insight into the mechanisms that underpin the functionality of this binary switch and elucidate its remarkable efficiency. The results also reveal novel SH3 binding surfaces highlighting the binding versatility and expanding the non-canonical ligand repertoire of this important signaling domain.