Tulp1 Is Involved in Specific Photoreceptor Protein Transport Pathways

Abstract

Tulp1 plays a critical role in protein transport from the photoreceptor inner segment (IS) to the outer segment (OS). To dissect which OS protein transport pathways are affected in the absence of Tulp1, we surveyed the localization of proteins destined for the OS in tulp1−/− mice. Immunohistochemistry was used to examine the localization of several classes of OS proteins as well as proteins involved in OS protein transport in young tulp1−/− mice prior to retinal degeneration. Comparisons were made to wild-type littermates. The absence of Tulp1 did not affect the transport of several phototransduction and OS structural proteins including phosphodiesterase, rhodopsin kinase, ROM-1, peripherin/RDS, and the cation channel. However, other phototransduction proteins such as rhodopsin, cone opsins, guanylate cyclase 1, and guanylate cyclase-activating proteins 1 and 2 were mislocalized to additional photoreceptor compartments. Two proteins that translocate in response to light stimulation were affected differently in tulp1−/− retinas; transducin translocated correctly whereas arrestin did not. In addition, chaperone proteins critical in the transport of rhodopsin-containing post-Golgi vesicles, Rab6, Rab8, and Rab11, were severely disrupted in tulp1−/− retinas. We conclude that Tulp1 is required for the correct transport of specific integral membrane proteins and their respective binding partners. Other classes of OS resident proteins do not appear to be affected. These differences support the hypothesis that Tulp1 plays a specific, critical role in photoreceptor OS protein transport pathways.