All lysosomal enzymes are translated on the endoplasmic reticulum (ER)-bound polyribosomes and contain a leader sequence. They enter the ER through the ER-specific translocons, and then lose the leader sequence. During translation the protein sequence, already within the ER is glycosylated on asparagine residues, which are part of the consensus sequence: Asparagine-X-Serine/Threonine (N-X-S/T; “X” can be any amino-acid except proline). These N-linked glycan trees (dubbed N-glycans) consist of nine mannose residues and5 three glucose residues. The enzymes are then folded, and, if recognized by the ER quality control machinery as correctly folded, they exit the ER in specific trafficking vesicles (called COPII vesicles) toward the Golgi network. They undergo further modifications on their N-linked glycan trees, which lose the glucose residues and three to four mannose residues. Most lysosomal enzymes are phosphorylated by the enzyme N-acetylglucosaminyl-1-phosphotransferase on a terminal mannose and as such are recognized by the mannose 6-phosphate receptor, which shuttles them, on specific late endosomal vesicles to the lysosomes. Mutations in the gene encoding this enzyme lead to severe LSD (I-cell disease), characterized by the abnormal secretion of all lysosomal enzymes whose lysosomal targeting depend on the M6P recognition signal.
If recognized as misfolded by the ER quality control machinery, lysosomal proteins undergo several folding attempts. If unsuccessful, the misfolded protein is translocated from the ER to undergo polyubiquitination and proteasomal degradation. This is known as ER-associated degradation (ERAD; Fig. 1). Persistent retention of misfolded molecules in the ER provokes ER stress and induces the Unfolded Protein Response, known as UPR.7 UPR monitors the conditions in the ER by sensing insufficiency in protein folding capacity and translating this information into gene expression by a signal transduction.
Fig1. THE ER-ASSOCIATED DEGRADATION (ERAD) PROCESS. Proteins in the secretory pathway are synthesized on ER bound ribosomes. Following their entry to the ER and loss of their leader peptide, they undergo glycosylation on asparagines. Recognition of the peptide as correctly folded allows its shuttling to the secretory pathway via the Golgi apparatus. When recognized as misfolded, the proteins undergo several chaperone-mediated attempts to refold them. If unsuccessful, the misfolded molecules are retro-translocated from the ER to the cytoplasm, where they are polyubiquitinated and undergo proteasomal degradation, known as ERAD.
Several lysosomal enzymes and proteins (including the hexosaminidase A alpha and beta chains, mutated in Tay-Sachs disease and in Sandhoff disease, respectively) undergo proteolytic cleavage in the lysosomes as the last stage of their maturation process.
A large number of mutant lysosomal enzymes undergo ERAD and, therefore, lysosomal diseases may be regarded as misfolding or conformational diseases.
