The inhibitory γ subunit of the rod cGMP phosphodiesterase binds the catalytic subunits in an extended linear structure

The unique feature of rod photoreceptor cGMP phosphodiesterase (PDE6) is the presence of inhibitory subunits (P gamma), which interact with the catalytic heterodimer (P alpha beta) to regulate its activity. This uniqueness results in an extremely high sensitivity and sophisticated modulations of rod visual signaling where the P gamma/P alpha beta interactions play a critical role. The quaternary organization of the alpha beta gamma gamma heterotetramer is poorly understood and contradictory patterns of interaction have been previously suggested. Here we provide evidence that supports a specific interaction, by systematically and differentially analyzing the P gamma-binding regions on P alpha and P beta through photolabel transfer from various P gamma positions throughout the entire molecule. The P gamma N- terminal Val(16)-Phe(30) region was found to interact with the P alpha beta GAFa domain, whereas its C terminus (Phe(73)-Ile(87)) interacted with the P alpha beta catalytic domain. The interactions of P gamma with these two domains were bridged by its central Ser(40)-Phe(50) region through interactions with GAFb and the linker between GAFb and the catalytic domain, indicating a linear and extended interaction between P alpha and P alpha beta. Furthermore, a photocross-linked product alpha beta gamma(gamma) was specifically generated by the double derivatized P gamma, in which one photoprobe was located in the polycationic region and the other in the C terminus. Taken together the evidence supports the conclusion that each P gamma molecule binds P alpha beta in an extended linear interaction and may even interact with both P alpha and P beta simultaneously.