Originally published as Genetics Published Articles Ahead of Print on February 16, 2005.

Genetics, Vol. 170, 263-273, May 2005, Copyright © 2005
doi:10.1534/genetics.104.036434

The Zebrafish pob Gene Encodes a Novel Protein Required for Survival of Red Cone Photoreceptor Cells

Michael R. Taylor*,1, Satoshi Kikkawa*,2, Antonio Diez-Juan{dagger}, Visvanathan Ramamurthy*, Koichi Kawakami{ddagger}, Peter Carmeliet{dagger} and Susan E. Brockerhoff*,3

* Department of Biochemistry, University of Washington, Seattle, Washington 98195
{dagger} Center for Transgene Technology and Gene Therapy, Flanders Interuniversity Institute for Biotechnology (VIB), University of Leuven, B-3000 Leuven, Belgium
{ddagger} Division of Molecular and Developmental Biology, National Institute of Genetics, Mishima, Shizuoka 411-8540, Japan

3 Corresponding author: Department of Biochemistry, Box 357350, University of Washington, Seattle, WA 98195.
E-mail: sbrocker{at}u.washington.edu

The zebrafish mutant, partial optokinetic response b (pob), was isolated using an N-ethyl N-nitrosourea (ENU)-based screening strategy designed to identify larvae with defective optokinetic responses in red but not white light. Previous studies showed that red-light blindness in pob is due to the specific loss of long-wavelength photoreceptor cells via an apoptotic mechanism. Here, we used positional cloning to identify the mutated pob gene. We find that pob encodes a highly conserved 30-kDa protein of unknown function. To demonstrate that the correct gene was isolated, we used the Tol2 transposon system to generate transgenic animals and rescue the mutant phenotype. The Pob protein contains putative transmembrane regions and protein-sorting signals. It is localized to the inner segment and synapse in photoreceptor cells, and when expressed in COS-7 cells it localizes to intracellular compartments. We also show that the degeneration of red cone photoreceptors in the mutants occurs independently of light. On the basis of our findings, we propose that Pob is not involved in phototransduction but rather plays an essential role in protein sorting and/or trafficking.