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Description

Introduction:

The secretory pathway is a series of events that allow the cell to make and secrete proteins. Secretory proteins are proteins that are made in the ER, processed in the Golgi. And transported into the lumen of the ER or the extracellular space. The secretory pathway represents the endoplasmic reticulum, the Golgi apparatus, and the passing vesicles. As well as the cell membranes and lysosomes. This is called the “secretory apparatus” because it is the cellular pathway that releases proteins into the extracellular medium.

But as always, the etymology tells only a small part of the story. This pathway processes membrane-binding proteins (either with cell membrane or ER or Golgi membrane). As well as lysosomal enzymes and any proteins that live in their own secret pathways.

Endoplasmic Reticulum:

The endoplasmic reticulum is the first step in the secretory pathway. The membrane continues with the outer atomic membrane. Although it is unclear why this is important because the protein does not begin to live in the nucleus. However, mRNA passes through the cytoplasm until it is taken up by the ribosome that wants to translate it. In “Translocation after translation”, new proteins switch to ER after translation. In the most interesting phenomenon known as “co-translational translocation”, ribosomes begin to translate like other proteins, but somewhere in the first 16 to 30 amino acids, they encounter signaling peptides (also known as signaling sequences). The signal motif is usually 1 positively charged amino acid followed by 6-12 hydrophobic amino acids.

This element is identified by signal recognition particles (SRPs, “ribonucleoproteins” or RNA / protein hybrid molecules) that bind and inhibit further translation by ribosomes. Translation stops until the ribosome / SRP complex encounters an SRP receptor on the ER membrane. When they meet, the SRP and receptor bind to each other with the GTP molecule on the ER membrane, apparently strengthening the interaction.

Fortunately, all of this happens with translocon Sec61, which forms channels across the ER layer. True transplantation is the synthesis of three different proteins (genes: SEC61A1 or SEC61A2, SEC61B, SEC61G), consisting of the Sec61a subunit 10 alpha helixes that pass through the membrane and form the channel. Once bound to the ribosome layer, translation continues, signaling peptides and eventually pushing all proteins through the channel into the ER lumen.

The sequence consists of two types of signal sequences:

The transfer termination sequence:

The transfer termination sequence (abbreviated as STA for a number of reasons) is a sequence of 22-25 hydrophobic amino acids that form the alpha helix somewhere in the center of the protein. When found, it is pushed into the membrane and the translation of other proteins continues into the cytosol. This “cancels” the translocation to the ER, which initiates the signaling peptide at the beginning of the protein (N-terminus).

The signal anchor sequence:

The signal anchor sequence (SA for short) is also a 22–25aa hydrophobic alpha helix, but on the left or right side contains 3 positively charged amino acids. Like the signaling peptide, it is detected by the SRP, which sends ribosomes to the ER. But unlike signaling peptides, this helical alpha sequence is embedded into the ER layer.

The insertion tendency is determined by the 3 positively charged amino acids. The positive charge must be on the cytosolic side, so that after the hydrophobic sequence (ie C-terminal), the protein ends with the C-terminal leading to the cytosol, but at the front (i.e. the N sequence of hydrophobic) with the N-terminal leading to the cytosol.

Protein translocation across the ER membrane:

The first stage of secretion is the transfer of proteins across the ER membrane. Protein translocation in ER takes place in a co-translational (ribosome-coupling) manner, where translation and translocation are directly linked or directly linked to translation and translocation (without ribosome-coupling) depending on the hydrophobic and amino acids of the existing signaling peptide. . Current composition. Fully translated. Zimmerman et al., 2011). Our general understanding is that both approaches use the same translocation channel, the Sec61 complex, and are bound to different channel partners.

Post-translational translocation:

Post-translational translocation (SRP-independent) has been shown to occur in all organisms, including mammalian cells, despite varying degrees. In rapidly evolving organisms, such as yeasts and bacteria, translocation is not always considered to follow translation (Rapoport, 2007), but in most eukaryotes, the post-translational mechanism is initially considered the translation route. Contrary to this hypothesis, different post-translational transport substrates are also found in mammalian cells (Lakkaraju et al., 2012).

Different preferences for co-translation and post-translation translation have also been observed in different East. In Saccharomyces cerevisiae, about 30% of the Sec61 complex is associated with ribosomes, whereas in Yarovia lipolytic, more than 70% of the complex is associated with ribosome translation.

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