SH3N and SH3C domain communication in chicken Crk II is mediated by interaction of CyPA. Prolyl cis-trans isomerization centered on P238FY regulates intradomain communication between the SH3N and SH3C domains. In the cis configuration, the SH3C or Crk forms a closed structure over the SH3N, preventing its association with proline-containing binding partners. In the trans configuration, the linker and SH3N are extended, releasing negative regulation. Blue = SH3N; Orange = SH3C; Green = CypA.