Phosducin (Pd) is a 28-kD phosphoprotein whose expression in retina
appears limited to photoreceptor cells. Pd binds to the β,γ
subunits of transducin (Gt). Their binding affinity is markedly
diminished by Pd phosphorylation. While Pd has long been regarded as a
candidate for the regulation of Gt, the molecular details of Pd
function remain unclear. This gap in understanding is due in part to a
lack of precise information concerning the total amount and subcellular
localization of rod Pd. While earlier studies suggested that Pd was a rod
outer segment (ROS) protein, recent findings have demonstrated that Pd is
distributed throughout the rod. In this report, the subcellular
distribution and amounts of rat Pd are quantified with immunogold electron
microscopy. After light or dark adaptation, retinal tissues were fixed
in situ and prepared for ultrathin sectioning and immunogold
labeling. Pd concentrations were analyzed over the entire length of the
rod. The highest Pd labeling densities were found in the rod synapse. Less
intense Pd staining was observed in the ellipsoid and myoid regions, while
minimal labeling densities were found in the ROS and the rod nucleus. In
contrast with rod Gt, no evidence was found for light-dependent
movement of Pd between inner and outer segments. There is a relative
paucity of Pd in the ROS as compared with the large amounts of
Gt found there. This does not support the earlier idea that Pd
could modulate Gt activity by controlling its concentration. On
the other hand, the presence of Pd in the nucleus is consistent with its
possible role as a regulator of transcription. The functions of Pd in the
ellipsoid and myoid regions remain unclear. The highest concentration of
Pd was found at the rod synapse, consistent with a suggested role for Pd
in the regulation of synaptic function.