The membrane-proximal region of the thrombopoietin receptor confers its high surface expression by JAK2-dependent and -independent mechanisms.
Janus tyrosine kinase 2 (JAK2) is essential for signaling by the thrombopoietin (TpoR) and erythropoietin (EpoR) receptors. In the absence of JAK2 most EpoR molecules are retained in the endoplasmic reticulum in an Endo H-sensitive form. In contrast, we show that in the absence of JAK2 a large fraction of the TpoR is processed to the mature Endo H-resistant form and reaches the cell surface. By studying chimeras of the TpoR and EpoR we show that high surface expression of the TpoR is entirely conferred by the membrane-proximal region of the intracellular domain that includes the juxtamembrane, Box 1, and Box 2 regions. The TpoR intracellular domain shows similar effects on receptor endocytosis rate as that of the EpoR, but does stabilize the mature receptor isoform from degradation. Co-expression of JAK2 further stabilizes mature TpoR and thus further increases its surface expression. This JAK2 effect depends on the Box 1 region, the only JAK2 interacting site in the TpoR. By contrast, EpoR requires Box 1 as well as the flanking 20 residues on the C-terminal side for JAK2 interaction and JAK2-dependent surface expression. Our study suggests that whereas cell surface expression of type I cytokine receptors requires their cognate JAKs, the mechanisms governing receptor-JAK interactions differ among receptors interacting with the same JAK protein.
Tong
Wei
W
Whitehead Institute for Biomedical Research, Cambridge, MA 02142, USA.
Sulahian
Rita
R
Gross
Alec W
AW
Hendon
Natalie
N
Lodish
Harvey F
HF
Huang
Lily Jun-shen
LJ
D000595:Amino Acid Sequence; D002454:Cell Differentiation; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D049109:Cell Proliferation; D002470:Cell Survival; D018076:DNA, Complementary; D004705:Endocytosis; D006412:Hematopoietic Stem Cells / Q000166:cytology / Q000378:metabolism; D006801:Humans; D053614:Janus Kinase 2 / Q000378:metabolism*; D008969:Molecular Sequence Data; D053628:Receptors, Thrombopoietin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid
D018076:DNA, Complementary; D053628:Receptors, Thrombopoietin; C507924:JAK2 protein, human; D053614:Janus Kinase 2