Carboxy terminally truncated forms of ribophorin I are degraded in pre-Golgi compartments by a calcium-dependent process.

Abstract

Two COOH terminally truncated variants of ribophorin I (RI), a type I transmembrane glycoprotein of 583 amino acids that is segregated to the rough portions of the ER and is associated with the protein-translocating apparatus of this organelle, were expressed in permanent HeLa cell transformants. Both variants, one membrane anchored but lacking part of the cytoplasmic domain (RL467) and the other consisting of the luminal 332 NH2-terminal amino acids (RI332), were retained intracellularly but, in contrast to the endogenous long lived, full length ribophorin I (t 1/2 = 25 h), were rapidly degraded (t 1/2 less than 50 min) by a nonlysosomal mechanism. The absence of a measurable lag phase in the degradation of both truncated ribophorins indicates that their turnover begins in the ER itself. The degradation of RI467 was monophasic (t 1/2 = 50 min) but the rate of degradation of RI332 molecules increased about threefold approximately 50 min after their synthesis. Several pieces of evidence suggest that the increase in degradative rate is the consequence of the transport of RI332 molecules that are not degraded during the first phase to a second degradative compartment. Thus, when added immediately after labeling, ionophores that inhibit vesicular flow out of the ER, such as carbonyl cyanide m-chlorophenylhydrazone (CCCP) and monensin, suppressed the second phase of degradation of RI332. On the other hand, when CCCP was added after the second phase of degradation of RI332 was initiated, the degradation was unaffected. Moreover, in cells treated with brefeldin A the degradation of RI332 became monophasic, and took place with a half-life intermediate between those of the two normal phases. These results point to the existence of two subcellular compartments where abnormal ER proteins can be degraded. One is the ER itself and the second is a non-lysosomal pre-Golgi compartment to which ER proteins are transported by vesicular flow. A survey of the effects of a variety of other ionophores and protease inhibitors on the turnover of RI332 revealed that metalloproteases are involved in both phases of the turnover and that the maintenance of a high Ca2+ concentration is necessary for the degradation of the luminally truncated ribophorin.