Quality control in the secretory assembly line

As a rule, only proteins that have reached a native, folded and assembled s tructure are transported to their target organelles and compartments within the cell. In the secretory pathway of eukaryotic cells, this type of sorti ng is particularly important. A variety of molecular mechanisms are involve d that distinguish between folded and unfolded proteins, modulate their int racellular transport, and induce degradation if they fail to fold. This phe nomenon, called quality control, occurs at several levels and involves diff erent types of folding sensors. The quality control system provides a strin gent and versatile molecular sorting system that guaranties fidelity of pro tein expression in the secretory pathway.