Nef has been shown to bind specifically to a subset of the Src family of kinases. Nef/Fyn interaction centers on a proline-rich motif (Pro-x-x-Pro), which is implicated in SH3 binding. This domain is partially disordered in the absence of the binding partner; when bound this motif fully adopts a left-handed polyproline type II helix conformation upon complex formation with the Fyn SH3 domain. Within this structure the arginine residue (Arg77) of Nef interacts with Asp 100 of the RT loop within the Fyn SH3 domain, and triggers a hydrogen-bond rearrangement which allows the loop to adapt to complement the Nef surface. The Arg96 residue of the Fyn SH3 domain is specifically accommodated in the same hydrophobic pocket of Nef. The Nef-Fyn complex forms in vivo and may have a crucial role in the T cell perturbating action of Nef by altering T cell receptor signaling.