Required for correct function and survival of retinal photoreceptors (PubMed:24098485, PubMed:24691551, PubMed:28475715). Required for retinal development (PubMed:28369466). In rod photoreceptors, facilitates stability and/or trafficking of guanylate cyclases and is required to maintain endoplasmic reticulum and mitochondrial homeostasis (PubMed:28475715). May play a role in clathrin-coated intracellular vesicle trafficking of proteins from the endoplasmic reticulum to the retinal rod plasma membrane (PubMed:24098485, PubMed:28369466). Interacts with STX3 (PubMed:28369466). Interacts with clathrin (PubMed:28369466). Expressed in the inner segment of rod photoreceptors and outer plexiform layer of the retina (at protein level) (PubMed:24691551, PubMed:28369466, PubMed:28475715). Expressed in liver, but not detected in brain, muscle, kidney, retinal cone photoreceptors or retinal ganglion cells (at protein level) (PubMed:24691551). Highly expressed in the ganglion cell layer of the retina and in liver, and also detected at low levels in kidney and testis (PubMed:15728532). Isoform 1: Expressed in the retina (PubMed:15728532, PubMed:24691551). Isoform 2: Expressed in liver (PubMed:24691551). Isoform 1 shows increasing levels of expression in the retina from birth, reaching maximal levels by 12 days of age. Viable, however mice develop progressive deterioration in retinal response, and male mice are sterile (PubMed:28369466, PubMed:28475715). Progressive retinal rod dysfunction first evident from one month of age progressing to almost undetectable scotopic response at one year of age (PubMed:28369466). Progressive degeneration of photopic response by retinal cone receptors first evident at one year of age (PubMed:28369466). Defective retinal morphology with significant thinning of the outer nuclear layer and reduced rows of nuclei (PubMed:28369466, PubMed:28475715). Accumulation of vacuole-like structures at the apical inner segment of the retina (PubMed:28369466). Reduced and disorganised photoreceptors with shortened fragmented outer segment (PubMed:28475715). Altered endoplasmic reticulum (ER) organization with increased ER area near the base of the outer segment, increased number of mitochondria in the rod ellipsoid region and induction of ER stress (PubMed:28475715). Reduced retinal expression of phototransduction proteins Cngb1 and Gnat1 and of Aipl1 (PubMed:28369466). Reduced expression of guanylate cyclases Gucy2e/GC1 and Gucy2f/GC2 in the rod outer segment and mislocalization of Pde6a from the outer segment to the inner segment and outer nuclear layer (PubMed:28475715). Belongs to the DP1 family.