The Golgi complex: structure and function
The above drawing shows an actual interface between the ER and the Golgi complex. The "Export complex" is seen at the top of the drawing. Note that the vesicle are moving to contribute to the cis-Golgi network of vesicles and cisternae.
The movement of these special transport vesicles is an energy requiring process. If one blocks production of ATP, the transport will not happen. This drawing shows how the rough endoplasmic reticulum forms vesicles (without ribosomes attached) that carry the newly synthesized proteins to the Golgi complex.
The inside of the vesicle becomes continuous with the inside of the Golgi cisternae, so that protein groups pointing towards the inside, could eventually be directed to face the outside of the cell.
Carbohydrate groups are attached and any subunits may be joined in these cisternae. The protein is then passed to the final region of the Golgi called the "trans face". There it is placed in vacuoles that bud from this region of the Golgi complex. These may be a certain size or density, characteristic of the cell itself. The vacuoles continue to condense the proteins and the final mature secretory granule is then moved to the membrane for secretion.
Some water soluble proteins gain their structure automatically (self-assembly) during synthesis. Proteins with complicated structures, many modifications, membrane-spanning domains can not be self-assembled. These proteins are modified step by step in the ER and the Golgi where many enzymes and chaperones help the proteins to get the active, three-dimensional structure.
This electron micrograph illustrates a Golgi complex. It is curved with its Trans face pointing away from the nucleus toward the cell periphery. The numerous vesicles in the area are transporting the proteins to and from cisternae.
Transport of material in and out of the Golgi complex involves budding and fusion of vesicles. This cartoon shows that the membranes of each join and align themselves during the process so that the inside face remains in the lumen and the outside face remains towards the cytoplasm.
The Golgi complex controls trafficking of different types of proteins. Some are destined for secretion. Others are destined for the extracellular matrix. Finally, other proteins, such as lysosomal enzymes, may need to be sorted and sequestered from the remaining constituents because of their potential destructive effects. This figure shows the two types of secretory pathways. The regulated secretory pathway, as its name implies, is a pathway for proteins that requires a stimulus or trigger to elicit secretion. Some stimuli regulate synthesis of the protein as well as its release. The constitutive pathway allows for secretion of proteins that are needed outside the cell, like in the extracellular matrix. It does not require stimuli, although growth factors may enhance the process.
Finally, this cartoon also shows the packaging of lysosomes which will be discussed in more detail in a later section.
Golgi complex regulate the insertion of plasma membrane proteins
Plasma membrane proteins are inserted in the membrane at the level of the rough endoplasmic reticulum. The protein sequence is coded for membrane insert start and stop sites. This directed the insertion and alignment points. Those that are multipass proteins have multiple start and stop sites. (we will discuss these questions at the lecture on Membrane and vesicular transport)