Microtubule-associated protein regulating the stability and length of the microtubule-based axoneme of photoreceptors. Required for the differentiation of photoreceptor cells, it plays a role in the organization of the outer segment of rod and cone photoreceptors ensuring the correct orientation and higher-order stacking of outer segment disks along the photoreceptor axoneme. Interacts (via the doublecortin domains) with microtubules. Interacts with RP1L1. Interacts with MAK. Specifically localized in the connecting cilia of rod and cone photoreceptors. Expressed in the cell bodies and inner segments of photoreceptors. Not found in liver, spleen, kidney, brain, thymus, muscle, heart, lung and testis. Gene expression is stimulated by retinal hypoxia and suppressed by relative retinal hyperoxia. The doublecortin domains, which mediate interaction with microtubules, are required for regulation of microtubule polymerization and function in photoreceptor differentiation. As early as postnatal day 7, mice have already undergone significant molecular retinal changes. The molecular responses change dramatically during development and were distinct from responses to the disruption of the photoreceptor transcription factors Crx, Pde6b and Nrl. The JNK signaling cascades are specifically compromised in Rp1 defective retinas. Double heterozygotes of Rp1 and Rp1l1 exhibit abnormal outer segment morphology and reduced single rod photosensitivity and dark currents.