A. Schmitz et al., IN-VIVO IODINATION OF A MISFOLDED PROINSULIN REVEALS CO-LOCALIZED SIGNALS FOR BIP BINDING AND FOR DEGRADATION IN THE ER, EMBO journal, 14(6), 1995, pp. 1091-1098
The signal for degradation of proteins in the endoplasmic reticulum (E
R) is thought to be the exposure of internal domains which are buried
when the protein has adopted its correct conformation and which are al
so exposed in assembly intermediates. This raises the question of why
the intermediates are not degraded. We developed a system based on the
peroxidase-catalyzed iodination of tyrosine residues which continuous
ly monitors the exposure of internal domains of proinsulin. In CHO cel
ls this system discriminated between assembly intermediates of wild ty
pe (wt) proinsulin and misfolded proinsulin, as shown by the exclusive
iodination of a misfolded mutant which was finally degraded in the ER
. Iodination in vitro showed that the assembly intermediates of wt pro
insulin also exposed internal domains. This iodination was inhibited b
y the addition of the molecular chaperone Bip which was co-immunopreci
pitated with proinsulin in CHO cells. The results obtained with the mu
tant proinsulin support the assumption that exposed internal domains r
epresent the signal for degradation in the ER. Observations of wt proi
nsulin show that Bip masks internal domains of normal assembly interme
diates during the entire assembly process, thereby suppressing their d
egradation. We propose that internal domains contain co-localized sign
als for Bip binding and for degradation.