In search of the identity of the XAP-1 antigen: a protein localized to cone outer segments.

Purpose: To determine the identity of the XAP-1 antigen. The XAP-1 antibody has been used by many investigators and is recognized as an index of photoreceptor outer segment maturity, yet its antigen remains unknown.

Methods: Previous studies documented that the XAP-1 antigen is a photoreceptor membrane-associated protein.

Downregulation of a unique photoreceptor protein correlates with improper outer segment assembly.

A unique photoreceptor protein has been characterized. This protein, termed XAP-1 antigen, is expressed by photoreceptors exclusively under conditions in which the outer segment membranes are properly assembled. When the retinal pigment epithelium is adherent to the underlying neural retina, the XAP-1 antigen is localized to the plasma membrane that surrounds the inner and outer segments in the areas juxtaposed to the subretinal space.