The GAFa domains of rod cGMP-phosphodiesterase 6 determine the selectivity of the enzyme dimerization

Retinal rod cGMP phosphodiesterase (PDE6 family) is the effector enzyme in the vertebrate visual transduction cascade. Unlike other known PDEs that form catalytic homodimers, the rod PDE6 catalytic core is a heterodimer composed of alpha and beta subunits. A system for efficient expression of rod PDE6 is not available. Therefore, to elucidate the structural basis for specific dimerization of rod PDE6, we constructed a series of chimeric proteins between PDE6alphabeta and PDE5, which contain the N-terminal GAFa/GAFb domains, or portions thereof, of the rod enzyme. These chimeras were co-expressed in Sf9 cells in various combinations as His-, myc-, or FLAG-tagged proteins. Dimerization of chimeric PDEs was assessed using gel filtration and sucrose gradient centrifugation. The composition of formed dimeric enzymes was analyzed with Western blotting and immunoprecipitation. Consistent with the selectivity of PDE6 dimerization in vivo, efficient heterodimerization was observed between the GAF regions of PDE6alpha and PDE6beta with no significant homodimerization. In addition, PDE6alpha was able to form dimers with the cone PDE6alpha' subunit. Furthermore, our analysis indicated that the PDE6 GAFa domains contain major structural determinants for the affinity and selectivity of dimerization of PDE6 catalytic subunits. The key dimerization selectivity module of PDE6 has been localized to a small segment within the GAFa domains, PDE6alpha-59-74/PDE6beta-57-72. This study provides tools for the generation of the homodimeric alphaalpha and betabeta enzymes that will allow us to address the question of functional significance of the unique heterodimerization of rod PDE6.