After entering the ER, newly synthesized proteins fold with the assistance of chaperones and folding enzymes, and their folding status is monitored by chaperones and also enzymes (Table 1).

Table1. Some Chaperones & Enzymes Involved in Folding That Are Located in the Rough Endoplasmic Reticulum

The chaperone calnexin is a calcium-binding protein located in the ER membrane. This protein binds a wide variety of proteins, including major histocompatibility complex (MHC) antigens and a variety of plasma proteins. As described in Chapter 46, calnexin binds the monoglucosylated species of glycoproteins that occur during processing of glycoproteins, retaining them in the ER until the glycoprotein has folded properly. Calreticulin, which is also a calcium-binding protein, has properties similar to those of calnexin, but it is not membrane-bound. In addition to chaperones, two enzymes in the ER lumen are concerned with proper folding of proteins. Protein disulfide isomerase (PDI) promotes rapid formation and reshuffling of disulfide bonds until the correct set is achieved. Peptidyl prolyl isomerase (PPI) accelerates folding of proline-containing proteins by catalyzing the cis–transisom erization of X-Pro bonds, where X is any amino acid residue.

Misfolded or incompletely folded proteins interact with chaperones, which retain them in the ER and prevent them from being exported to their final destinations. If such interactions continue for a prolonged period of time, harmful buildup of misfolded proteins is avoided by ER-associated degradation (ERAD). In a number of genetic diseases, such as cystic fibrosis, misfolded proteins are retained in the ER, and in some cases, the retained proteins still exhibit some functional activity. As discussed later in this chapter, drugs that will interact with such proteins and promote their correct folding and export out of the ER are currently under investigation.

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مواضيع ذات صلة

Proteoglycans & Glycosaminoglycans

Proteins Follow Several Routes to be Inserted Into or Attached to The Membranes of The Endoplasmic Reticulum

Proteins sorted via the Rough ER branch have N-Terminal signal peptides

Proteins Imported Into Peroxisomes Carry Unique Targeting Sequences

Transport of Macromolecules in & out of the Nucleus Involves Localization Signals

Most Mitochondrial Proteins Are Imported

Many proteins are targeted by signal sequences to their correct destinations

Conjugation reactions in phase 2 metabolism prepare Xenobiotics for excretion

Isoforms of Cytochrome P450 Hydroxylate a Wide Variety of Xenobiotics in Phase 1 Metabolism

Glycans are involved in The Binding of Viruses, Bacteria, & Some Parasites to Human Cells

Glycoproteins are Involved in Many Biological Processes & in Many Diseases

The Biosynthesis of N-Linked Glycoproteins Involves Dolichol-P-P-Oligosaccharide