The Endoplasmic Reticulum or ER is an extensive system of internal membranes that move proteins and other substances through the cell. The part of the ER with attached ribosomes is called the rough ER. The rough ER helps transport proteins that are made by the attached ribosomes.
The Golgi apparatus transports and modifies proteins in eukaryotic cells. The Golgi apparatus is the central organelle mediating protein and lipid transport within the eukaryotic cell.
1. Most organelle proteins are made on free ribosomes and then imported (post-translationally) into the organelles. Proteins are imported by passing through pores or transport complexes (translocases) in the organelle membranes. See Becker 22-18 (20-18).
Carrier proteins and channel proteins are the two major classes of membrane transport proteins. Carrier proteins (also called carriers, permeases, or transporters) bind the specific solute to be transported and undergo a series of conformational changes to transfer the bound solute across the membrane (Figure 11-3).
The endoplasmic reticulum (ER ) is an organelle that helps make and transport proteins and lipids. There are two types of endoplasmic reticulum: rough endoplasmic reticulum ( RER ) and smooth endoplasmic reticulum (SER). Both types are shown in the figure below. RER looks rough because it is studded with ribosomes.
The components of the plasma membrane
|Phospholipids||Main fabric of the membrane|
|Cholesterol||Tucked between the hydrophobic tails of the membrane phospholipids|
|Integral proteins||Embedded in the phospholipid bilayer; may or may not extend through both layers|
Ribosomes are the sites in a cell in which protein synthesis takes place. Cells have many ribosomes, and the exact number depends on how active a particular cell is in synthesizing proteins. For example, rapidly growing cells usually have a large number of ribosomes (Figure 5).
Proteins destined to be secreted move through the secretory pathway in the following order: rough ER → ER-to-Golgi transport vesicles → Golgi cisternae → secretory or transport vesicles → cell surface (exocytosis) (see Figure 17-13). Small transport vesicles bud off from the ER and fuse to form the cis-Golgi reticulum.
Proteins are translocated into the mitochondrial matrix space by passing through the TOM and TIM complexes at sites of adhesion between the outer and inner membranes known as contact sites. Some imported proteins also contain an internal signal sequence that guides their further transport.
Ovalbumin 1ova (1.0Mb) [Bbk|BNL|ExP|Waw|Hal] is an example of a secretory protein which does not naturally have its signal sequence cleaved. The 100 N-terminal residues are found to be necessary for transport through the membrane to be effected. All nuclear proteins are synthesised on free ribosomes in the cytoplasm.
Protein synthesis occurs in the cytoplasm on ribonucleoprotein particles, the ribosomes.
Aquaporins (AQP) are integral membrane proteins that serve as channels in the transfer of water, and in some cases, small solutes across the membrane. They are conserved in bacteria, plants, and animals. Structural analyses of the molecules have revealed the presence of a pore in the center of each aquaporin molecule.
Transport proteins are integral transmembrane protein; that is they exist permanently within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion or active transport.
Collagen is a special case and is used as a model protein for studying protein transport; not only is collagen the most abundant structural protein in vertebrates, but it is too large to be accommodated within conventional transport vesicles.