Differential expression of rod photoreceptor cGMP-phosphodiesterase alpha and beta subunits: mRNA and protein levels.

The catalytic core of photoreceptor-specific cGMP-phosphodiesterase (PDE) consists of two subunits, PDEalpha and PDEbeta, that are homologous and have similar domain organization but are encoded by different genes. We have examined the PDEalpha and PDEbeta mRNA steady-state and protein levels as well as the biosynthesis rate of these proteins in developing and fully differentiated retinas. We have also determined the translational efficiency of PDE subunits and the role of their mRNA structures in regulating protein synthesis. In mature retinas, PDEalpha and PDEbeta are represented by approximately 1.5 x 108 and 7.5 x 108 copies/microg retinal mRNA, respectively. The levels of these transcripts in developing photoreceptors (P10) are approximately 75% of those at P30. Quantification of protein concentration indicated that PDEalpha and PDEbeta are equally expressed in developing and fully differentiated photoreceptors. Furthermore, the PDEalpha/PDEbeta ratios obtained throughout a 2-h pulsechase period revealed a similar turnover rate for both subunits. The observed discordance between the mRNA and protein levels of PDEalpha and PDEbeta suggested post-transcriptional regulation of their expression. We found that PDEalpha mRNA is translated more efficiently than either of the two PDEbeta transcripts expressed in retina. Therefore, the lower level of PDEalpha mRNA is compensated by its more efficient translation to achieve equimolar expression with PDEbeta. We also analyzed the effect of PDEalpha and PDEbeta mRNA 5'- and 3'-untranslated regions as well as that of their coding regions on protein synthesis. We determined that the PDE-coding regions play a critical role in the differential translation of these subunits.