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Vol. 17, Issue 5, 2243-2255, May 2006

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Luteinizing Hormone Receptor Ectodomain Splice Variant Misroutes the Full-Length Receptor into a Subcompartment of the Endoplasmic Reticulum

Pirjo M. Apaja ^* , Jussi T. Tuusa ^* , E. Maritta Pietilä ^* , Hannu J. Rajaniemi  , and Ulla E. Petäjä-Repo ^*  

^* Biocenter Oulu, University of Oulu, FI-90014 Oulu, Finland; Department of Anatomy and Cell Biology, University of Oulu, FI-90014 Oulu, Finland

Submitted September 20, 2005; Revised February 6, 2006; Accepted February 13, 2006
Monitoring Editor: Jeffrey Brodsky

The luteinizing hormone receptor (LHR) is a G protein–coupled receptor that is expressed in multiple RNA messenger forms. The common rat ectodomain splice variant is expressed concomitantly with the full-length LHR in tissues and is a truncated transcript corresponding to the partial ectodomain with a unique C-terminal end. Here we demonstrate that the variant alters the behavior of the full-length receptor by misrouting it away from the normal secretory pathway in human embryonic kidney 293 cells. The variant was expressed as two soluble forms of M_r 52,000 and M_r 54,000, but although the protein contains a cleavable signal sequence, no secretion to the medium was observed. Only a very small fraction of the protein was able to gain hormone-binding ability, suggesting that it is retained in the endoplasmic reticulum (ER) by its quality control due to misfolding. This was supported by the finding that the variant was found to interact with calnexin and calreticulin and accumulated together with these ER chaperones in a specialized juxtanuclear subcompartment of the ER. Only proteasomal blockade with lactacystin led to accumulation of the variant in the cytosol. Importantly, coexpression of the variant with the full-length LHR resulted in reduction in the number of receptors that were capable of hormone binding and were expressed at the cell surface and in targeting of immature receptors to the juxtanuclear ER subcompartment. Thus, the variant mediated misrouting of the newly synthesized full-length LHRs may provide a way to regulate the number of cell surface receptors.

Abbreviations used: BSA, bovine serum albumin; CG, chorionic gonadotropin; DDM; n-dodecyl--D-maltoside; Endo H, endo--N-acetylglucosaminidase H; ER, endoplasmic reticulum; ERAD, ER-associated degradation; GPCR, G protein–coupled receptor; HA, haemagglutinin; HEK293, human embryonic kidney 293; LH, luteinizing hormone; µOR, µ opioid receptor; PNGase F, peptide-N-glycosidase F; R, receptor; STI, soybean trypsin inhibitor; UPR, unfolded protein response.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

These authors contributed equally to this work.

Address correspondence to: Ulla Petäjä-Repo (Ulla.Petaja-Repo{at}oulu.fi) or Hannu Rajaniemi (Hannu.Rajaniemi{at}oulu.fi).

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