Jx. Zhang et al., QUALITY-CONTROL IN THE SECRETORY PATHWAY - THE ROLE CALRETICULIN, CALNEXIN AND BIP IN THE RETENTION OF GLYCOPROTEINS WITH C-TERMINAL TRUNCATIONS, Molecular biology of the cell, 8(10), 1997, pp. 1943-1954
Unlike properly folded and assembled proteins, most misfolded and inco
mpletely assembled proteins are retained in the endoplasmic reticulum
of mammalian cells and degraded without transport to the Golgi complex
. To analyze the mechanisms underlying this unique sorting process and
its fidelity, the fate of C-terminally truncated fragments of influen
za hemagglutinin was determined An assortment of different fragments w
as generated by adding puromycin at low concentrations to influenza vi
rus-infected tissue culture cells. Of the fragments generated, < 2% wa
s secreted, indicating that the system for detecting defects in newly
synthesized proteins is quite stringent. The majority of secreted spec
ies corresponded to folding domains within the viral spike glycoprotei
n. The retained fragments acquired a partially folded structure with i
ntrachain disulfide bonds and conformation-dependent antigenic epitope
s. They associated with two lectin-like endoplasmic reticulum chaperon
es (calnexin and calreticulin) but not BiP/GRP78. Inhibition of the as
sociation with calnexin and calreticulin by the addition of castanospe
rmine significantly increased fragment secretion. However, it also cau
sed association with BiP/GRP78. These results indicated that the assoc
iation with calnexin and calreticulin was involved in retaining the fr
agments. They also suggested that Bip/GRP78 could serve as a backup fo
r calnexin and calreticulin in retaining the fragments. In summary, th
e results showed that the quality control system in the secretory path
way was efficient and sensitive to folding defects, and that it involv
ed multiple interactions with endoplasmic reticulum chaperones.