hello and welcome to cell clips today we will be talking about the transport of proteins into the endoplasmic reticulum or er if you've seen other cell clips videos you know that proteins synthesized in the cytoplasm either remain there or get transported to the mitochondria the nucleus for the ER however there are other locations within the cell such as the cell membrane endosomes and lysosomes as well as destinations outside of the cell where proteins need to be transported proteins destined for these other locations first pass through the ER where they get modified folded and packaged into vesicles that fuse with the Golgi apparatus in the Golgi the proteins are further modified and packaged into vesicles that get sorted to various locations in this cell we'll talk more about these processes in future cell clips videos but today we will focus on how these proteins get imported into the ER there are two modes of transport for proteins going into the ER post-translational translocation and Co translational translocation let's take a closer look at each of these processes individually post-translational translocation looks similar to the protein import that occurs in other organelles in this mode of transport proteins are synthesized completely by free ribosomes in the cytoplasm bound by chaperones that prevent them from folding and then import it into the ER through a channel form 2 pi translocon called sex 61 these proteins have an ER signal sequence or zip code that is recognized and bound by this X 61 translocon with the help of accessory proteins this signal sequence can be located anywhere on the peptide and is composed of a region of hydrophobic amino acids flanked by two hydrophilic regions when the protein binds this x61 translocon a small part of the protein extends into the ER lumen this region is immediately bound by a protein called pip which prevents the protein from slipping back out of the channel particles in a fluid are subject to random movement called Brownian motion so the protein jostles around in the channel this causes new regions to emerge in the ER lumen and as soon as a new portion of the protein enters the ER bit put binds and prevents it from reversing back out over time the jostling of the peptide and the binding of bit pulls the peptide into the ER this kind of transport system is called a ratchet and shows up in other types of protein transport in the cell in this case pulling the peptide into the ER does not require energy but releasing the strong bond between BIP and the protein uses energy from ATP hydrolysis though post-translational translocation is similar to other types of protein transport it is not the most common form of import into the ER the more common mode is code translational translocation which involves the transport of proteins into the ER as they are being synthesized this is possible because the ER has ribosomes covering its surface that can synthesize proteins directly into the sex 61 channel so translation and translocation happen concurrently like proteins imported post-translationally these proteins have a zip code made of a region of hydrophobic amino acids with hydrophilic regions on either end however unlike post translational transport this sequence is almost always found at the end terminus and is cleaved from all secreted proteins and many transmembrane proteins by protein called signal peptidase so this sequence may not appear in the final protein encodes lai tional transport the signal sequence is first bound by a protein called the signal recognition particle or SRP SRP has three binding domains the first domain binds the signal sequence as soon as it is translated the second binds the ribosome stopping translation and the third binds an SRP receptor located in the ER membrane near this x61 channel at this point this ecstasy one channel binds the signal sequence causing SRP to dissociate once SRP disengages the ribosome continues translating the peptide directly into the ER lumen as you can see this xst one translocon plays an important role in protein transport across the ER membrane but this is not it's only function the sexy one channel has a hinge that can open during translocation causing the protein to slip out and insert into the ER membrane this happens whenever a transmembrane domain enters the channel transmembrane domains are hydrophobic regions about twenty to twenty-five amino acids long all proteins destined for the ER have at least one of these domains because the signal sequence itself is hydrophobic however transmembrane proteins often have multiple transmembrane domains once inserted into the ER membrane these proteins maintain their orientation throughout their transport in the ER and Golgi this means that the same region of the protein facing the cytosol in the ER will face the cytosol when it arrives at its final destination therefore it is essential that these domains are inserted into the ER membrane with a very specific orientation this orientation is determined by the charge of the amino acids on either side of the signal sequence the ER lumen stores calcium ions which gives it a net positive charge while the cytosol maintains a net negative charge positively charged amino acids will preferentially orient towards the cytosol while negatively charged amino acids will be positioned in the yard lumen it might help to think of this like a needle and thread if you're familiar with sewing you know that you usually start by tying a knot you thread the needle through the fabric in a direction that places that not on a specific side of the fabric then the thread is pulled through when the needle passes through the fabric again it causes a loop of string to form on one side this process repeats in the site the thread is on alternates with each stitch this is similar to the multi pass membrane protein in this analogy the n-terminus of the protein is like the knot it is positioned so that it extends either into the ER or into the cytosol anchoring the protein on the membrane then the growing peptide extends out like a thread until a second transmembrane domain slips out of the channel forming a loop the peptide then begins extending on the opposite side of the membrane and this alternating pattern continues each time a transmembrane domain enters the channel like the placement of the knot in the sewing analogy the orientation of the first hydrophobic domain determines the orientation of all subsequent transmembrane passes so in summary proteins can be transported into the ER one of two ways code translationally or post translationally in post-translational translocation peptides are synthesized completely in the cytoplasm bound by chaperones and then translocated into the ER by a ratchet system encode translational translocation newly forming peptides are quickly bound by SRP which stops translation and brings them to the sex 61 channel these peptides then bind 661 SRP is released and translation continues directly into the ER transmembrane domains slip out of the channel and these domains maintain the same orientation in the membrane throughout their transport in the ER and all G look for other cell clips videos to learn more about what happens next as these proteins are folded modified and transported to their final destination that's it for this video thanks for watching Cell clips