Abstract
There are fundamental differences in the structures of outer segments between rod and cone photoreceptor cells in the vertebrate retina. Visual pigments are the only essential membrane proteins that differ between rod and cone outer segments, making it likely that they contribute to these structural differences. Human rhodopsin is N-glycosylated on Asn2 and Asn15, whereas human (h) red and green cone opsins (hOPSR and hOPSG, respectively) are N-glycosylated at Asn34 Here, utilizing a monoclonal antibody (7G8 mAB), we demonstrate that hOPSR and hOPSG from human retina also are O-glycosylated with full occupancy. We determined that 7G8 mAB recognizes the N-terminal sequence 21DSTQSSIF28 of hOPSR and hOPSG from extracts of human retina, but only after their O-glycans have been removed with O-glycosidase treatment, thus revealing this post-translational modification of red and green cone opsins. In addition, we show that hOPSR and hOPSG from human retina are recognized by jacalin, a lectin that binds to O-glycans, preferentially to Gal-GalNAc. Next, we confirmed the presence of O-glycans on OPSR and OPSG from several vertebrate species, including mammals, birds, and amphibians. Finally, the analysis of bovine OPSR by MS identified an O-glycan on Ser22, a residue that is semi-conserved (Ser or Thr) among vertebrate OPSR and OPSG. These results suggest that O-glycosylation is a fundamental feature of red and green cone opsins, which may be relevant to their function or to cone cell development, and that differences in this post-translational modification also could contribute to the different morphologies of rod and cone photoreceptors.
Highlights
There are fundamental differences in the structures of outer segments between rod and cone photoreceptor cells in the vertebrate retina
The analysis of bovine OPSR by Mass spectrometry (MS) identified an O-glycan on Ser22, a residue that is semi-conserved (Ser or Thr) among vertebrate OPSR and OPSG. These results suggest that O-glycosylation is a fundamental feature of red and green cone opsins, which may be relevant to their function or to cone cell development, and that differences in this post-translational modification could contribute to the different morphologies of rod and cone photoreceptors
It is unlikely that Thr36 is O-glycosylated as N-glycosylated Asn34 would be expected to act as an obstruction and because Thr36 is part of the N-glycosylation motif (NXT) that is strictly conserved in vertebrate OPSR/OPSG
Summary
The analysis of bovine OPSR by MS identified an O-glycan on Ser, a residue that is semi-conserved (Ser or Thr) among vertebrate OPSR and OPSG These results suggest that O-glycosylation is a fundamental feature of red and green cone opsins, which may be relevant to their function or to cone cell development, and that differences in this post-translational modification could contribute to the different morphologies of rod and cone photoreceptors. Mass spectrometry (MS) analysis of bovine OPSR identified Ser as the O-glycosylation site, a residue that is semi-conserved (Ser or Thr) among vertebrates, suggesting an important role of O-glycosylation, perhaps in the unique features of green and red cone photoreceptors. Our findings show that OPSG heterologously expressed in mammalian and insect cells is O-glycosylated but has a glycan pattern and/or occupancy that is different from that observed in retina, emphasizing that heterologous expression systems are often unable to mimic the PTMs observed in native tissues
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