Fig. (5) Molecular mechanisms of GRK1 activation. GRK1 is a membrane – associated enzyme that has low affinity for Rho. After photoactivation, due to conformational changes in the receptor, GRK1 forms a complex with Rho* by associating with the secondo and third cytoplasmic loops of the receptor. According to one model, GRK1 becomes activated, dissociates, and phosphorilates the C - terminal region of many Rho. According to a second model, GRK1 remains bound and phosphorilates the nearby C – termini of Rho and Rho* at Ser³³⁴, Ser³³⁸ or Ser³⁴³.