Rod outer segment-associated N-acetylgalactosaminylphosphotransferase

Area of Science:

• Ophthalmology
• Cell Biology
• Biochemistry

Background:

• N-acetylgalactosaminylphosphotransferase (GalNAcPTase) is a glycosyltransferase.
• This enzyme has been associated with cell adhesion molecules like N-cadherin and E-cadherin in other tissues.
• Its presence and function in mammalian rod outer segments (ROS) were previously unknown.

Purpose of the Study:

• To determine the precise location of GalNAcPTase in mammalian ROS.
• To confirm if anti-GalNAcPTase antibodies recognize retinal molecules with transferase activity.
• To characterize the enzyme activity and its acceptors within ROS.

Main Methods:

• Immunoelectron microscopy using anti-GalNAcPTase antibodies on fixed ROS.
• Protein fractionation, gel electrophoresis, and Western blotting of retinal and ROS proteins.
• Enzyme assays to characterize transferase activity and identify endogenous acceptors.

Main Results:

• GalNAcPTase was localized to the ROS cell surface and associated with the interphotoreceptor matrix (IPM).
• The antibody recognized a ~220 kDa protein with transferase activity in Western blots.
• ROS GalNAcPTase exhibited activity towards very high molecular mass endogenous acceptors, likely IPM proteoglycans.

Conclusions:

• GalNAcPTase is present on ROS and in the IPM, associated with the cell surface.
• The enzyme appears to glycosylate itself and other proteins, particularly large IPM molecules.
• The ROS GalNAcPTase may play a role in modulating cell adhesion within the retina.