The GTP- and Phospholipid-Binding Protein TTD14 Regulates Trafficking of the TRPL Ion Channel in Drosophila Photoreceptor Cells

PLoS Genet. 2015 Oct 28;11(10):e1005578. doi: 10.1371/journal.pgen.1005578. eCollection 2015 Oct.

Abstract

Recycling of signaling proteins is a common phenomenon in diverse signaling pathways. In photoreceptors of Drosophila, light absorption by rhodopsin triggers a phospholipase Cβ-mediated opening of the ion channels transient receptor potential (TRP) and TRP-like (TRPL) and generates the visual response. The signaling proteins are located in a plasma membrane compartment called rhabdomere. The major rhodopsin (Rh1) and TRP are predominantly localized in the rhabdomere in light and darkness. In contrast, TRPL translocates between the rhabdomeral plasma membrane in the dark and a storage compartment in the cell body in the light, from where it can be recycled to the plasma membrane upon subsequent dark adaptation. Here, we identified the gene mutated in trpl translocation defective 14 (ttd14), which is required for both TRPL internalization from the rhabdomere in the light and recycling of TRPL back to the rhabdomere in the dark. TTD14 is highly conserved in invertebrates and binds GTP in vitro. The ttd14 mutation alters a conserved proline residue (P75L) in the GTP-binding domain and abolishes binding to GTP. This indicates that GTP binding is essential for TTD14 function. TTD14 is a cytosolic protein and binds to PtdIns(3)P, a lipid enriched in early endosome membranes, and to phosphatidic acid. In contrast to TRPL, rhabdomeral localization of the membrane proteins Rh1 and TRP is not affected in the ttd14P75L mutant. The ttd14P75L mutation results in Rh1-independent photoreceptor degeneration and larval lethality suggesting that other processes are also affected by the ttd14P75L mutation. In conclusion, TTD14 is a novel regulator of TRPL trafficking, involved in internalization and subsequent sorting of TRPL into the recycling pathway that enables this ion channel to return to the plasma membrane.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell Membrane / genetics
  • Cell Membrane / metabolism
  • Darkness
  • Drosophila Proteins / genetics*
  • Drosophila Proteins / metabolism
  • Drosophila melanogaster
  • Eye / metabolism
  • GTP-Binding Proteins / genetics*
  • Light
  • Membrane Proteins / genetics*
  • Membrane Proteins / metabolism
  • Mutation
  • Photoreceptor Cells, Invertebrate / metabolism*
  • Protein Transport / genetics*
  • Rhodopsin / metabolism
  • Signal Transduction
  • Transient Receptor Potential Channels / genetics*
  • Transient Receptor Potential Channels / metabolism

Substances

  • Drosophila Proteins
  • Membrane Proteins
  • Transient Receptor Potential Channels
  • trpl protein, Drosophila
  • Rhodopsin
  • GTP-Binding Proteins
  • TTD14 protein, Drosophila

Grants and funding

This project was funded by grants provided to Uwe Wolfrum and Armin Huber. Uwe Wolfrum was funded by Deutsche Forschungsgemeinschaft (WO548/8-1); the FAUN-Stiftung, Nurnberg; European Community FP7/2009/241955 (SYSCILIA) and the BMBF, grant 0314106 (HOPE2). Armin Huber received funding from the Deutsche Forschungsgemeinschaft (Hu 839/2-5; Hu 839/2-6 and Hu 839/7-1). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.