Polarized sorting of rhodopsin on post-Golgi membranes in frog retinal photoreceptor cells

We have isolated a subcellular fraction of small vesicles (mean diameter, 300 nm) from frog photoreceptors, that accumulate newly synthesized rhodopsin with kinetics paralleling its appearance in post-Golgi membranes in vivo. This fraction is separated from other subcellular organelles including Golgi and plasma membranes and synaptic vesicles that are sorted to the opposite end of the photoreceptor cell. The vesicles have very low buoyant density in sucrose gradients (rho = 1.09 g/ml), a relatively simple protein content and an orientation of rhodopsin expected of transport membranes. Reversible inhibition of transport by brefeldin A provides evidence that these vesicles are exocytic carriers. Specific immunoadsorption bound vesicles whose protein composition was indistinguishable from the membranes sedimented from the subcellular fraction. Some of these proteins may be cotransported with rhodopsin to the rod outer segment; others may be involved in vectorial transport.