Summary
Protein C (PC) deficiency and plasmin inhibitor (PI) deficiency are inherited thrombotic
and haemorrhagic disorders. We investigated the intracellular degradation of mutant
proteins, using naturally occurring PC and PI mutants that lead to congenital deficiencies.
To examine the necessity of N-linked glycosylation for the proteasomal degradation
of PC and PI, PC178 and PC331 mutants treated with tunicamycin and N-glycosylation-lacking
mutants, PC92Stop and PI-America were pulse chased. The analysis revealed that the
speed of degradation of the tunicamycin-treated PC mutants, PC92Stop and PI-America
lacking glycosylation, was slower than that of N-glycosylated mutants. Immunoprecipitation
and immunoblot analysis showed that PC178 and PC331 mutants were associated with molecular
chaperones, Bip, GRP94, and calreticulin. PI-America was associated with only Bip.
Although degradation of mutants was mediated by proteasomes, no association with ubiquitin
was detected. Co-transfection of endoplasmic reticulum (ER) degradation enhancing
α-mannosidase-like protein (EDEM) accelerated the degradation of N-glycosylated PC.
In the absence of autophagy using Atg5-deficient cell lines, the degradation of the
PC331 mutant was mildly accelerated but that of PC178, PI-America and PI-Okinawa mutants
was not influenced. While the degradation of the PC and PI mutants was facilitated
by N-glycosylation moieties, they were ubiquitin-independently degraded by proteasomes,
irrespective of the presence or absence of N-glycosylation. Molecular chaperone binding
was influenced by the presence of N-glycosylation moieties. When the misfolded or
truncated mutant proteins are functionally active, proteasome inhibitors such as bortezomib
may have therapeutic potential for treatment of protein deficiencies.
Keywords
Endoplasmic reticulum-associated degradation - N-linked glycosylation - protein C
- plasmin inhibitor - proteasome inhibitor