TMEM67, TMEM237 and embigin in the complex with lactate transporter MCT1 are unique components of the photoreceptor outer segment plasma membrane.

The outer segment (OS) organelle of vertebrate photoreceptors is a highly specialized cilium evolved to capture light and initiate light response. The plasma membrane which envelopes the outer segment plays vital and diverse roles in supporting photoreceptor function and health. However, little is known about the identity of its protein constituents, as this membrane cannot be purified to homogeneity. In this study, we employed the technique of protein correlation profiling to identify unique OS plasma membrane proteins. To achieve this, we used label-free quantitative mass spectrometry to compare relative protein abundances in an enriched preparation of the OS plasma membrane to a preparation of total outer segment membranes. We have found that only five proteins were enriched at the same level as previously validated OS plasma membrane markers. Two of these proteins, TMEM67 and TMEM237, had not been previously assigned to this membrane and one, embigin, had not been identified in photoreceptors. We further showed that embigin associates with monocarboxylate transporter MCT1 in the OS plasma membrane, facilitating lactate transport through this cellular compartment.