Expression and subcellular distribution of UNC119a, a protein partner of transducin α subunit in rod photoreceptors

Abstract

A recently discovered interaction of rod transducin α subunit (Gαt1) with UNC119a is thought to be important for transducin trafficking in photoreceptors. In this study, we analyzed the subcellular distribution of UNC119a under different conditions of illumination in vivo. Analyses by immunofluorescence and Western blotting of retina serial tangential sections demonstrated that UNC119a resides predominantly in the rod inner segment, with a small fraction of UNC119a also appearing to infiltrate the rod outer segment. Such a distribution is consistent with the proposed role of UNC119a in facilitating transducin transport from the rod inner segment to the outer segment in the dark. In addition, UNC119a was present in smaller amounts in the cell body and synaptic region of rods. The profile of UNC119a subcellular distribution remained largely unchanged under all tested conditions of illumination, and correlated with the profile of Gαt1 following its light-dependent translocation. Quantification by Western blotting suggested that mouse retina contains ~17pmol of UNC119a, giving a ~1 to 4 molar ratio of UNC119a to Gαt1. Hence, light-translocated Gαt1 can serve as a major partner of UNC119a. Supporting this role, the levels of UNC119a were downregulated by about 2-fold in mouse retina lacking Gαt1. As a dominant partner, Gαt1 may potentially modulate the function of other known UNC119a-interacting proteins involved in photoreceptor ciliary trafficking and synaptic regulation, in a light-dependent manner.