Receptor-transporting protein (RTP) family members play divergent roles in the functional expression of odorant receptors

PLoS One. 2017 Jun 6;12(6):e0179067. doi: 10.1371/journal.pone.0179067. eCollection 2017.

Abstract

Receptor transporting protein (RTP) family members, RTP1S and RTP2, are accessory proteins to mammalian odorant receptors (ORs). They are expressed in the olfactory sensory neurons and facilitate OR trafficking to the cell-surface membrane and ligand-induced responses in heterologous cells. We previously identified different domains in RTP1S that are important for different stages of OR trafficking, odorant-mediated responses, and interaction with ORs. However, the exact roles of RTP2 and the significance of the requirement of the seemingly redundant co-expression of the two RTP proteins in vivo have received less attention in the past. Here we attempted to dissect the functional differences between RTP1S and RTP2 using a HEK293T cell-based OR heterologous expression system. When a set of 24 ORs were tested against 28 cognate ligands, unlike RTP1S, which always showed a robust ability to support odorant-mediated responses, RTP2 had little or no effect on OR responses and exhibited a suppressive effect over that of RTP1S for a subset of the ORs tested. RTP1S and RTP2 showed no significant difference in OR ligand selectivity and co-transfection with RTP2 increased the detection threshold for some ORs. A protein-protein interaction analysis showed positive interactions among OR, RTP1S, and RTP2, corroborating the functional linkages among the three molecules. Finally, further cell-surface and permeabilized immunocytochemical studies revealed that OR and the co-expressed RTP1S proteins were retained in the Golgi when co-transfected with RTP2, indicating that RTP1S and RTP2 could play different roles in the OR trafficking process. By examining the functional differentiations between the two RTP family members, we provided a molecular level explanation to the suppressive effect exerted by RTP2, shedding light on the divergent mechanisms underlying the RTP proteins in regulating the functional expression of ORs.

MeSH terms

  • Golgi Apparatus / metabolism
  • HEK293 Cells
  • Humans
  • Membrane Transport Proteins / genetics*
  • Membrane Transport Proteins / metabolism
  • Olfactory Receptor Neurons / metabolism
  • Protein Interaction Maps / genetics
  • Protein Transport / genetics
  • Receptors, Odorant / genetics*
  • Receptors, Odorant / metabolism
  • Transfection

Substances

  • Membrane Transport Proteins
  • RTP1 protein, human
  • RTP2 protein, human
  • Receptors, Odorant

Grant support

The work was supported by the Chinese National Science Foundation (31070972), Science and Technology Commission of Shanghai Municipality (16ZR1418300), the Program for Innovative Research Team of Shanghai Municipal Education Commission, the Shanghai Eastern Scholar Program (J50201), and the National Basic Research Program of China (2012CB910401). All of the funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.