Sialoglycoproteins of the frog rod outer segment plasma membrane

Photoreceptors can segregate inner segment proteins from outer segment proteins. It is uncertain whether such a sorting process occurs during disc morphogenesis in rods, thereby resulting in an outer segment plasma membrane which differs compositionally from the discs. In this study, the plasma membranes of intact, purified frog rod outer segments (ROS) have been labeled with a peanut lectin (PNA), either with or without prior neuraminidase treatment of the ROS. Neuraminidase removes terminal sialic acid residues from membrane-bound sialoglycoconjugates. PNA was demonstrated to bind to the plasma membrane of ROS only after neuraminidase treatment, as detected by fluorescence light microscopy and electron microscopy. In order to identify the sialoglycoproteins responsible for this labeling, ROS proteins were resolved by polyacrylamide gel electrophoresis and transferred to nitrocellulose paper for incubation with PNA. A single, prominent band at 125 Kd bound PNA after neuraminidase treatment of ROS. This band represents a protein which is an integral component of the ROS plasma membrane and is not associated with the interphotoreceptor matrix or other potential contaminants. Selective degradation of ROS prior to neuraminidase treatment indicates that this sialoglycoprotein is absent from discs and, thus, can serve as a marker specific for ROS plasma membrane during fractionation studies.