In vitro isoprenylation and membrane association of mouse rod photoreceptor cGMP phosphodiesterase alpha and beta subunits expressed in bacteria

We investigated the specificity of CAAX box-related isoprenylation of rod photoreceptor cGMP phosphodiesterase (PDE) subunits expressed in bacteria and the consequences of this modification on rod disk membrane association. Full-length cDNA sequences of the alpha and beta subunits of mouse PDE, inserted into bacterial pET expression vectors, were overexpressed as fusion proteins containing 28 (bMP-alpha) and 26 (bMP-beta) additional amino acid residues at their N termini. Both fusion proteins were overexpressed and stored in inclusion bodies. Purified bMP-alpha and bMP-beta were recognized by bovine PDE-specific polyclonal antibodies, but did not associate with depleted rod disk membranes and were catalytically inactive. Using bovine brain or retina extracts as sources of protein prenyltransferases and tritiated farnesyl- or geranylgeranylpyrophosphate as donors, bMP-alpha (CAAX sequence CCIQ) was exclusively farnesylated, and bMP-beta (CAAX sequence CCIL) was exclusively geranylgeranylated. After isoprenylation, bMP-alpha and bMP-beta each associated with rod photoreceptor outer segment disk membranes under isotonic, but not under hypotonic, conditions. The results indicate that isoprenylated bMP-alpha and bMP-beta each interact independently with membranes and that isoprenylation is the key modification that facilitates membrane association.