 |
 |
|
|
PP2A Photoreceptor signal transduction
|
Protein Phosphatase Type 2ABackground: PP2A is an abundant serine/threonine phosphatase involved in many other cellular events besides the phototransduction pathway. For example, PP2A plays a role in the critical cellular processes of protein synthesis, DNA replication, transcription, and metabolism. PP2A is composed of three protein subunits, A, B, and C. Subunit 'A' is a 60-65 kDa structural component, 'C' is a 36-38 kDa catalytic subunit, and 'B' is a 54-130 kDa regulatory subunit. While there are multiple isoforms of the C subunit (Ca and Cb) and A subunit (Aa and Ab), the B subunit is the most variable with many different isoforms-including B' and B''. In addition, the B' isoform has several subtypes, including B'a. Many of the B isoforms display greater than 80% sequence identity. The core complex of PP2A is comprised of the A and C subunits, which are tightly associated. This dimeric core can be complexed with the regulatory B subunit, which alters the substrate specificity of the PP2A holoenzyme. Specific B subunits are known to be selectively expressed in various locations, for example the B56a and B56g are highly expressed in heart and skeletal muscle while B56b is selectively expressed in the brain. Zhao et al. have shown that the various B subunits are not redundant, but are involved in equipping the enzyme for different cellular functions. For example, the Bg subunit allows PP2A to target specific cytoskeletal structures within brain neurons. Preliminary studies from the Mary D. Allen Laboratories for Vision Research show that the presence of a B' subunit determines the role of PP2A in rod photoreceptor cells, and that the presence of a B' subunit is light/dark dependent. Specifically, the AC dimer of PP2A dephosphorylates rhodopsin in the dark, while the AC-B' trimer actively dephosphorylates phosducin in the light. |
