Biological Membranes - A. N. Ogurtsov 2012

Structure and Functions of Biomembrans
Intracellular Protein Transport
Transmembrane Translocation

Protein transport into Cell/35.html">Mitochondria. Mitochondria contain their own DNA and synthesize some of the Proteins required for their function; however, the majority of mitochondrial proteins are encoded by The Cell's nuclear genome. Such proteins are synthesized on Ribosomes in the Cytosol and then transported into the mitochondrion. As noted above, proteins destined for the mitochondrial matrix contain a specific targeting sequence at their N-terminus. Mitochondrial receptor proteins recognize this sequence and bind to a translocation complex, which unfolds the protein globule and threads the polypeptide chain through both mitochondrial membranes. Following translocation, the targeting sequence is cleaved off, and the protein folds into its native functional globule.

Molecular chaperone proteins actively assist in unfolding the protein globule prior to translocation and during secondary folding, thereby protecting the polypeptide chain from nonspecific aggregation.

Transport into Peroxisomes. In most Organelles bounded by a single membrane, proteins are delivered via Vesicular Transport (Figure 76). Peroxisomes are an exception to this rule.

Peroxisomal proteins are synthesized in the cytosol and subsequently transported into peroxisomes. Peroxisomal targeting Amino acid sequences within these proteins bind to import receptor proteins in the cytosol. These transport protein-receptor complexes then attach to the peroxisomal membrane, cross the membrane, and release the transported protein into the peroxisomal lumen, while the receptors are recycled back to the cytosol.

Protein Synthesis in the rough Endoplasmic reticulum.

The presence of ribosomes on the membranes of the rough endoplasmic reticulum clearly indicates that this is a site of protein synthesis. However, in most cases, the proteins synthesized on the ribosomes of the rough ER do not participate in any intracellular processes within that given cell—they are "useless" to it, and sometimes even harmful.

For example, the mammary gland Cells synthesize large amounts of milk casein on their rough endoplasmic reticulum ribosomes, which is entirely unnecessary for the gland cells themselves. Similarly, the ribosomes of the rough ER in digestive gland cells produce hydrolytic Enzymes; if these enzymes were to leak into the cytosol, it would inevitably lead to self-Digestion and cell death.

However, this does not happen because the synthesized proteins are translocated across the membrane of the rough endoplasmic reticulum into its interior (lumen), thereby becoming immediately isolated from the cytosol and cytoplasmic structures.

Thus, the function of the rough endoplasmic reticulum is not merely to synthesize proteins on the ribosomes of its membranes, but also to isolate these proteins from the rest of the cell's cytosolic proteins.

A GENERALIZED SCHEME OF METABOLISM/35.html">Protein Biosynthesis is presented in Figure 82.

Protein biosynthesis (mRNA Translation) always begins in the cytosol (Figure 82 (1)). A specific sequence of 15–60 Amino Acids at the beginning of the chain, known as a signal peptide, determines the site of synthesis. If the protein emerging from the ribosome begins with a signal peptide (Figure 82 (2)) that directs it to the rough endoplasmic reticulum, an RNA-containing signal-recognition particle (SRP) binds to it, and translation is temporarily halted (Figure 82 (3)).

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Figure 82 - Protein synthesis in the rough endoplasmic reticulum

The SRP binds the ribosome via an SRP receptor to the membrane of the rough endoplasmic reticulum (Figure 82 (4)). As soon as the ribosome attaches to the membrane, the SRP dissociates from both the signal peptide and the SRP receptor (accompanied by GTP Hydrolysis), and translation resumes on the ribosome (Figure 82 (5)).

The protein chain elongates on the ribosome and, still unfolded, passes through the membrane via a channel called a translocon into the lumen of the rough endoplasmic reticulum (Figure 82 (6)).

Upon completion of translation, the signal peptide of the secretory protein is cleaved off by a specialized enzyme, leader peptidase, preventing the protein from leaving the lumen of the rough endoplasmic reticulum. This mechanism achieves the vectorial discharge of proteins.

The passage of the growing polypeptide through the membrane can be interrupted by a specific stop-transfer signal. In this case, the polypeptide remains embedded in the membrane, giving rise to an integral membrane protein. During protein synthesis, the growing chain may pass through the membrane multiple times, with synthesis reinitiation mediated by a signal peptide. A membrane protein produced via this mechanism will contain multiple transmembrane domains.

Most proteins synthesized on the membranes of the rough endoplasmic reticulum are Glycoproteins, whereas soluble cytosolic proteins are unglycosylated. Glycosylation is another vital biosynthetic function of The endoplasmic reticulum. This glycosylation influences the subsequent fate of the proteins.