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 reticul um and then diverted to a degradative pathway by a network of molecular cha perones and intracellular proteases, Here we report that mutant insulin pro receptors (pro(62)) retained in the early secretory pathway undergo proteol ytic cleavage at a tetrabasic concensus site for the subtilisin-like protea se furin (SPC 1), generating two unstable proteolytic intermediates of 80/1 20 kDa corresponding to alpha>(*) over bar * (135 kDa) and beta (90 kDa) su bunits. These are degraded more rapidly than the uncleaved proreceptor prot ein. 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 increas ed the degradation of mutant but not wildtype receptors. Together, these re sults suggest that processing and degradation occur sequentially for mutant proreceptors.