CYSTOL HAD HIGHER PH THAN ER LUMEN SERIES

The translocon is actually a complex of three different proteins (genes: SEC61A1 or SEC61A2, SEC61B, SEC61G), of which the Sec61a subunit has 10 membrane-spanning a-helices which form the channel. Fortuitously, this all happens adjacent to a Sec61 translocon – a protein complex that forms a channel crossing the ER membrane. When they meet, SRP and its receptor each bind one GTP molecule in the ER membrane, which apparently strengthens their interaction. Translation is stopped until the ribosome/SRP complex encounters an SRP receptor on the ER membrane. This motif gets recognized by signal recognition particle (SRP, a ‘ribonucleoprotein’ or hybrid RNA/protein molecule) which binds to it and prevents the ribosome from continuing translation. That signal’s motif is often 1 positively charged amino acid followed by 6-12 hydrophobic amino acids. In the more interesting phenomenon called ‘cotranslational translocation’ the ribosome starts translation just like any other protein, but somewhere in the first 16 to 30 amino acids it hits a signal peptide (aka signal sequence). In ‘posttranslational translocation’ the new protein is moved into the ER after it’s translated. Rather, mRNAs drift around in the cytoplasm until they get picked up by a ribosome interested in translating them. Its membrane is continuous with the outer nuclear membrane, though it’s not clear why that matters, since it’s not like proteins begin their life in the nucleus. The endoplasmic reticulumis the first step in the secretory pathway. This is beautifully depicted in the Life of the Cell video: Today’s lecture will focus on how proteins get translated into the ER and how they travel (in vesicles) between the ER, Golgi and other destinations.

CYSTOL HAD HIGHER PH THAN ER LUMEN SERIES

Many of them need chaperones to help with folding, and/or a whole series of post-translational modifications in order to be ready for their native function, and the secretory pathway specializes in providing them all of that. Many proteins that go through the secretory pathway never touch the cytosol – except the parts of membrane proteins that stick out on the cytosolic side. The secretory pathway is not contiguous, but every movement between its components is in little bubbled-off microcosms of its own chemical world, called vesicles. Hepatocytes (in the liver) sequester drugs and toxins in the smooth ER and break them down for excretion from the body there. The secretory pathway provides a route for the cell to handle things that might not be good to have in the cytoplasm, and/or are most useful when kept concentrated in a specialized compartment with their desired interacting partners. Moreover, different proteins may live only in the secretory pathway or only in the cytosol. This makes for different protein-folding conditions: for instance, disulfide bonds usually only form in oxidative conditions. The cytosol is reductive (when you’re in the cytosol, you keep meeting molecules that want to offer you electrons), and the ER, Golgi and extracellular environment are oxidative (molecules keep coming up to you asking for electrons). The cytosol and the ‘lumen’ (the liquid that fills the secretory pathway) are different chemical environments, and they normally never mix. It also does some things other than process proteins. This pathway also processes proteins that will be membrane-bound (whether in the cellular membrane or in the ER or Golgi membranes themselves), as well as lysosomal enzymes, and also any proteins that will live their lives in the secretory pathway itself. But as usual, etymology only tells a fraction of the story. It’s named ‘secretory’ for being the pathway by which the cell secretes proteins into the extracellular environment. The secretory pathwayrefers to the endoplasmic reticulum, Golgi apparatus and the vesicles that travel in between them as well as the cell membrane and lysosomes. These are notes from lecture 4 of Harvard Extension’s Cell Biology course.

Cystol had higher ph than er lumen