ZIP3, a new splice variant of the PKC-zeta-interacting protein family, binds to GABAC receptors, PKC-zeta, and Kv beta 2.

The correct targeting of modifying enzymes to ion channels and neurotransmitter receptors represents an important biological mechanism to control neuronal excitability. The recent cloning of protein kinase C-zeta interacting proteins (ZIP1, ZIP2) identified new scaffolds linking the atypical protein kinase PKC-zeta to target proteins. GABA(C) receptors are composed of three rho subunits (rho 1-3) that are highly expressed in the retina, where they are clustered at synaptic terminals of bipolar cells. A yeast two-hybrid screen for the GABA(C) receptor rho 3 subunit identified ZIP3, a new C-terminal splice variant of the ZIP protein family. ZIP3 was ubiquitously expressed in non-neuronal and neuronal tissues, including the retina. The rho 3-binding region of ZIP3 contained a ZZ-zinc finger domain, which interacted with 10 amino acids conserved in rho 1-3 but not in GABA(A) receptors. Consistently, only rho 1-3 subunits bound to ZIP3. ZIP3 formed dimers with ZIP1-3 and interacted with PKC-zeta and the shaker-type potassium channel subunit Kv beta 2. Different domains of ZIP3 interacted with PKC-zeta and the rho 3 subunit, and simultaneous assembly of ZIP3, PKC-zeta and rho 3 was demonstrated in vitro. Subcellular co-expression of ZIP3 binding partners in the retina supported the proposed protein interactions. Our results indicate the formation of a ternary postsynaptic complex containing PKC-zeta, ZIP3, and GABA(C) receptors.