Ligand-independent growth hormone receptor dimerization occurs in the endoplasmic reticulum and is required for ubiquitin system-dependent endocytosis

^*Department of Cell Biology and Institute of Biomembranes, University Medical Center, 3584 CX, Utrecht, The Netherlands; and ^†Departments of Pediatrics, Cell Biology, and Physiology, Washington University School of Medicine, St. Louis, MO 63110

Communicated by Harvey F. Lodish, Massachusetts Institute of Technology, Cambridge, MA (received for review December 23, 2001)

Abstract

The regulatory effect of growth hormone (GH) on its target cells is mediated via the GH receptor (GHR). GH binding to the GHR results in the formation of a GH-(GHR)₂ complex and the initiation of signal transduction cascades via the activation of the tyrosine kinase JAK2. Subsequent endocytosis and transport to the lysosome of the ligand-receptor complex is regulated via the ubiquitin system and requires the presence of an intact ubiquitin-dependent endocytosis (UbE) motif in the cytosolic tail of the GHR. Recently, the model of ligand-induced receptor dimerization has been challenged. In this study, ligand-independent GHR dimerization is demonstrated in the endoplasmic reticulum and at the cell surface by coimmunoprecipitation of an epitope-tagged truncated GHR with wild-type GHR. In addition, evidence is provided that the extracellular domain of the GHR is not required to maintain this interaction. Internalization of a chimeric receptor, which fails to dimerize, is independent of an intact UbE-motif. Therefore, we postulate that dimerization of GHR molecules is required for ubiquitin system-dependent endocytosis.

Footnotes

↵ ‡ To whom reprint requests may be addressed. E-mail: strous{at}med.uu.nl.
Abbreviations:

GH,

growth hormone;

GHR,

GH receptor;

TMD,

transmembrane domain;

ER,

endoplasmic reticulum;

HA,

hemagglutinin;

LRP,

low-density lipoprotein receptor-related protein;

RAP,

receptor-associated protein;

Epo,

erythropoietin;

EpoR,

Epo receptor;

wt,

wild type;

UbE,

ubiquitin-dependent endocytosis;

LRP4,

LRP ligand-binding domain 4