Final answer:
Integral membrane proteins with stop transfer sequences and soluble proteins destined for secretion or other organelles are transferred to the ER. These proteins include those that embed within the ER membrane or move further along the endomembrane system after post-translational modification, such as folding or glycosylation.
Step-by-step explanation:
Types of Proteins Transferred to the ER
Proteins that are transferred from the cytosol to the endoplasmic reticulum (ER) include integral membrane proteins and soluble proteins that are to be secreted out of the cell or delivered to other organelles. When these proteins are synthesized by ribosomes in the cytosol, they have signal sequences that direct their insertion into the ER membrane. The ER is particularly involved in the post-translational modification of proteins; this includes folding, glycosylation (the addition of sugarSide chains), and assembly into functional protein complexes.
Integral membrane proteins typically have a stop transfer sequence that halts their translocation into the ER, causing them to become embedded within the membrane. These proteins are then incorporated into the ER membrane or transported via vesicles to other membranes within the cell, including the Golgi apparatus, lysosomes, or the plasma membrane.
In the case of ER-bound ribosomes, they facilitate the co-translational transfer of newly synthesized proteins into the lumen of the RER, where they undergo subsequent modifications, and in some cases, contribute to the synthesis of phospholipids for cellular membranes.