Tubby-like Protein 1 (TULP1) Interacts with F-actin in Photoreceptor Cells

TULP1 is a member of the TULP family of four proteins (TUB and TULP1, -2, and -3), characterized structurally by the highly conserved C-terminal half of the protein.1 TULP proteins are localized primarily to nervous tissues with TUB and TULP3, which are widely distributed throughout the central nervous system, and TULP1 and TULP2, which are restricted largely to the retina and testis, respectively.1–6 In the retina, TULP1 is found exclusively in photoreceptor cells, localizing primarily in the inner segments and connecting cilium and to a lesser extent in the perinuclear cytoplasm and synaptic termini.6–9 The functions of these proteins are not known; however, the strong conservation of TULP proteins through evolution suggests that they function in a basic pathway in cells in which they are expressed. Other than members of the TULP family, database searches do not reveal significant homology with known proteins or functional motifs. TULP proteins play a central role in cellular function, as two of the family members have been linked to neurosensory disease phenotypes. Mice homozygous for a mutation in the Tub gene exhibit retinal and cochlear degeneration as well as adult-onset obesity associated with insulin resistance.4,10,11 Mutations in human TULP1 have been shown to cause retinitis pigmentosa (RP), a heterogeneous group of inherited retinal diseases in which the rod and cone photoreceptor cells degenerate, leading to blindness.12–15 Unlike the tubby phenotype in mice, patients with TULP1 mutations are not obese and have no hearing impairment; however, they have a more severe visual handicap in comparison to patients with RP caused by defects in other retinal genes.12,16 Similarly, loss of TULP1 function in mice, introduced by gene knockout, also causes photoreceptor degeneration.7,9 The degeneration in mice resembles the phenotypic characteristics reported in patients with RP due to TULP1 mutations, in that it is an early-onset, progressive, panretinal degeneration involving both rod and cone photoreceptor cells. Given their association with ocular disease, the predominant quest surrounding TULP proteins is to determine their physiological roles. Toward this end, there have been several studies performed to identify the function of TUB, as it is the founding member of the TULP family and its phenotype involves both neurosensory and neuroendocrine deficits. A wide array of postulated cellular functions include vesicular trafficking,17 mediation of insulin signaling,18 gene transcription,19 G-protein signaling,20 and ribosomal RNA synthesis.5 However, there is no direct evidence pertaining to the function of TULP1. As a step toward understanding how TULP1 maintains the health of photoreceptor cells, we determined the subcellular localization of TULP1 and pursued the identification of TULP1 interacting proteins in the retina. In the present study, we showed that TULP1 is a cytoplasmic protein that associates with membranes through binding phospholipids and provide evidence that TULP1 interacts with the cytoskeletal protein F-actin. We also demonstrated that TULP1 and actin colocalize in photoreceptor cells of the retina and in cultured COS7 cells. These data provide the first direct evidence that TULP1 may be involved in actin cytoskeletal functions, such as membrane trafficking or protein movement, and support our previous hypothesis that TULP1 may be involved in photoreceptor protein trafficking.