Bulk flow revisited: transport of a soluble protein in the secretory pathway.
Traffic. 2009 Dec; 10(12):1819-30
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Using a rapidly folding, non-glycosylated viral protein, the authors measure the rate and efficiency with which this foreign protein traverses the secretory pathway. Secretion is detected within 15 minutes after synthesis, is faster than endogenous proteins, and is highly efficient. These data argue against a strict requirement for a receptor-based recognition mechanism for rapid and efficient endoplasmic reticulum (ER) export.
Protein export from the ER utilizes a quality control machinery to ensure that only properly folded proteins are released. Rate-limiting steps for protein secretion are the folding and assembly steps that include disulfide bond formation, glycosylation, and multimeric association. Thor et al. present a novel system that enables them to monitor the progress of a completely foreign protein through the ER and Golgi of Chinese hamster ovary (CHO) and Madin-Darby canine kidney (MDCK) cells. They follow the export of an HA-tagged, signal sequence-bearing, C-terminal domain of the Semliki Forest virus capsid protein that is normally cytoplasmic in origin. The protein is known to fold co-translationally and in a chaperone-independent manner; it is also an auto-protease, and its self-cleavage can be used to report on its folding completion. The protein folds within 1 minute, appears in the medium linearly from 15 minutes to two hours, and 85% is released. It uses the normal secretory pathway because secretion is blocked by Brefeldin-A and requires ATP. Current models for ER export invoke sorting receptors, and there exist several well-established examples of coat-cargo or lectin-cargo interactions that facilitate ER export in specific cases. But it is difficult to imagine receptors for all cargoes, and this study suggests that receptors may not always be required. The absence of a receptor requirement would help explain the ability of eukaryotic cells to secrete foreign proteins, including green fluorescent protein (GFP), bacterial beta-lactamase and, from this elegant work, Semliki Forest virus capsid protein.
Pfeffer S: F1000Prime Recommendation of [Thor F et al., Traffic 2009, 10(12):1819-30]. In F1000Prime, 16 Nov 2009; DOI: 10.3410/f.1168077.630195. F1000Prime.com/1168077#eval630195
F1000Prime Recommendations, Dissents and Comments for [Thor F et al., Traffic 2009, 10(12):1819-30]. In F1000Prime, 23 Apr 2014; F1000Prime.com/1168077
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