The regulatory subunit of PDE6 interacts with PACSIN in photoreceptors

PURPOSE

In photoreceptors, phosphodiesterase 6 (PDE6) is regulated in response to light, due to the shuttling of a regulatory subunit (PDE6gamma) between the catalytic subunits of PDE6 and the activated form of transducin. We showed previously that PDE6gamma is able to interact with the Src-homology type 3 (SH3) domain of formin-binding protein 17 (FBP17), a protein involved in membrane receptor endocytosis. FBP17 was not detected in rat retina. Therefore, we looked for other SH3 domain-containing proteins that might interact with PDE6gamma in rat photoreceptors.

METHODS

Several SH3 domains highly homologous to this domain of FBP17 were found by structural alignment. Yeast two-hybrid system and GST pull-downs were used to test interaction of PDE6gamma with these putative partners. Expression patterns in rat retina of the SH3 containing candidates were also determined by immunohistochemistry and western blotting. GST pull-downs and co-immunopreciptations were then used to test in vivo interaction with PDE6gamma in rat retina extracts. Colocalization and light translocation of PDE6gamma and one of its partner were studied by confocal microscopy.

RESULTS

PDE6gamma interacts in vitro with a number of SH3 domains. These interactions involve a polyprolin motif located between amino acids 20 and 28 of PDE6gamma. Several of the putative partners of PDE6gamma are expressed in photoreceptor cells and might therefore interact in vivo with PDE6gamma. Our results show that only PACSIN, a protein implicated in endocytosis, was found to interact with PDE6gamma in rat retina extracts. The colocalization of the two proteins occurs in photoreceptor inner segments and synapses and is greatly enhanced upon illumination of the retina.

CONCLUSIONS

PDE6gamma function is mostly documented in the regulation of phototransduction. Our results provide evidence that in vitro PDE6gamma has a broad pattern of SH3 containing partners expressed in photoreceptors. PDE6gamma interaction with PACSIN points to a possible role of PDE6gamma in endocytosis. Further studies will be needed to understand the exact role of PDE6gamma-PACSIN interactions in photoreceptors. The description of this new function of PDE6gamma might help to understand the molecular mechanism of the severe retinal degeneration observed in PDE6gamma knock-out mice.

A proline-rich domain in the gamma subunit of phosphodiesterase 6 mediates interaction with SH3-containing proteins

PURPOSE

Phosphodiesterase 6 (PDE6) is the primary effector of phototransduction in vertebrate photoreceptors. Previous studies described the expression of the regulatory subunit of rod PDE6 (Pgamma-rod) in non-photosensitive tissues of the adult rat and the effects of this protein on MAP kinase pathways. Upon examination of the Pgamma-rod sequence, we detected a proline-rich domain that might reveal its ability to interact with SH3-containing proteins. Therefore, the present study was initiated to identify new protein partners of Pgamma-rod.

METHODS

A yeast two-hybrid screen of a rat brain cDNA library was performed using Pgamma-rod as a bait. Pgamma-rod-SH3 interaction was confirmed by GST pull-down of in vitro-translated proteins. The aminoacids involved in the interaction were mapped by site-directed mutagenesis. Rnase protection assay, RT-PCR and western blot analysis were used to detect Pgamma-rod expression in various rat tissues.

RESULTS

A clone was isolated twice, that consisted essentially of the SH3 domain of the formin-binding protein 17 (FBP17). This interaction was confirmed by GST pull-down. Mutational analysis of the Pgamma-rod-FBP17 interaction confirmed it involved the proline-rich domain of Pgamma-rod and the SH3 domain of FBP17. This proline-rich domain also allowed Pgamma-rod to interact with Cdc42-interacting protein 4 (CIP4), another SH3-containing protein. RT-PCR and Rnase protection assay detected different amounts of Pgamma-rod mRNA in adult and embryonic rat tissues. Western blots confirmed the presence of low levels of Pgamma-rod protein only in embryonic tissues.

CONCLUSIONS

Our data suggest that Pgamma-rod participates in SH3-mediated cellular pathways and may therefore play a wider role than previously appreciated. One possibility is that FBP17 interaction with sorting nexin 2 might connect Pgamma-rod to receptor tyrosine kinase recycling. However, further studies are still required to identify the diversity of SH3-containing proteins that interact with Pgamma-rod. This effort should provide a rationale to understand how Pgamma-rod can affect receptor internalization-dependent MAP kinase activity.