Furin-mediated processing in the early secretory pathway: sequential cleavage and degradation of misfolded insulin receptors.
Improperly folded membrane proteins are retained in the endoplasmic reticulum and then diverted to a degradative pathway by a network of molecular chaperones and intracellular proteases. Here we report that mutant insulin proreceptors (Pro(62)) retained in the early secretory pathway undergo proteolytic cleavage at a tetrabasic concensus site for the subtilisin-like protease furin (SPC 1), generating two unstable proteolytic intermediates of 80/120 kDa corresponding to alpha (135 kDa) and beta (90 kDa) subunits. These are degraded more rapidly than the uncleaved proreceptor protein. Site-directed mutagenesis of the normal RKRR processing site prevented cleavage. Use of inhibitors and furin-deficient cell lines confirmed that furin is responsible for proreceptor cleavage; furin overexpression increased the degradation of mutant but not wild-type receptors. Together, these results suggest that processing and degradation occur sequentially for mutant proreceptors.
Bass
J
J
Howard Hughes Medical Institute, Chicago, IL 60637, USA. j-bass@northwestern.edu
Turck
C
C
Rouard
M
M
Steiner
D F
DF
D001483:Base Sequence; D002460:Cell Line; D017931:DNA Primers; D004721:Endoplasmic Reticulum / Q000378:metabolism; D045683:Furin; D006801:Humans; D006868:Hydrolysis; D018832:Molecular Chaperones / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D017510:Protein Folding; D011972:Receptor, Insulin / Q000235:genetics / Q000378:metabolism*; D013381:Subtilisins / Q000378:metabolism*
D017931:DNA Primers; D018832:Molecular Chaperones; D011972:Receptor, Insulin; D013381:Subtilisins; D045683:Furin