Grb14 is a member of the Grb7 family of adapters and acts as a negative regulator of insulin-mediated signaling. Here we found that the protein kinase C (PKC) interacting protein, ZIP, interacted with Grb14. Coimmunoprecipitation experiments demonstrated that ZIP bound to both Grb14 and PKC, thereby acting as a link in the assembly of a PKC-ZIP-Grb14 heterotrimeric complex. Mapping studies indicated that ZIP interacted through its ZZ zinc finger domain with the phosphorylated insulin receptor interacting region (PIR) of Grb14. PKC phosphorylated Grb14 under in vitro conditions and in CHO-IR cells as demonstrated by in vivo labeling experiments. Furthermore, Grb14 phosphorylation was increased under insulin stimulation, suggesting that the PKC-ZIP-Grb14 complex is involved in insulin signaling. The PIR of Grb14, which also interacts with the catalytic domain of the insulin receptor (IR) and inhibits its activity, was preferentially phosphorylated by PKC. Interestingly, the phosphorylation of Grb14 by PKC increased its inhibitory effect on IR tyrosine kinase activity in vitro. The role of ZIP and Grb14 in insulin signaling was further investigated in vivo in Xenopus laevis oocytes. In this model, ZIP potentiated the inhibitory action of Grb14 on insulininduced oocyte maturation. Importantly, this effect required the recruitment of PKC and the phosphorylation of Grb14, providing in vivo evidences for a regulation of Grb14-inhibitory action by ZIP and PKC. Together, these results suggest that Grb14, ZIP, and PKC participate in a new feedback pathway of insulin signaling.