The Cytoskeleton and Cell Trafficking
In the last lesson, we learned about the various organelles to include the Golgi apparatus. As a quick recap, the Golgi apparatus is the shipping center of the cell and doesn’t just freely release packaged products into the cytoplasm. Instead, it uses the cytoskeleton and its various motor proteins to transport them to their intended destination.
The Cytoskeleton
While the cytoskeleton plays an important role in cell trafficking it also provides structure and support to the cell much as our bones do. In order to carry out these functions, the cytoskeleton utilizes three different components, microfilaments, intermediate filaments, and microtubules each with slightly different roles.
Microfilaments
The smallest of them all is the microfilament. These filaments are made up of actin monomers (G-actin) which polymerize into long filamentous chains (F-actin).
As a result of their structure microfilaments are excellent at resisting compressive forces and function to protect the cell from various mechanical forces. Furthermore, they provide cells the ability to move by using ATP in conjunction with myosin to generate muscular contraction as we will explore later.
Intermediate Filaments
Moving up on the size scale are the intermediate filaments. Unlike microfilaments, the intermediate filaments consist of various filamentous proteins including keratin, desmin, lamins, and more. Despite this diversity, they only have two major roles, cell to cell adhesion, and structural support.
For now, that is all we need to know and we will revisit the intermediate filaments when we discuss tissues and how the cells within them are held together.
Microtubule
Last and biggest of the three is the microtubule. Microtubules are the train tracks of the cell and are composed of two monomers called ⍺-tubulin and β-tubulin, which polymerize to form long hollow tube structures that span the cell.
They are organized into bundles called centrioles in the centrosome region of the cell and play an important role in mitosis and meiosis.
With the help of kinesin and dynein, motor proteins that act like cargo-carrying trains, microtubules are able to transport vesicles throughout the cell. This allows for secretory molecules to make it to the cell membrane, lipids to enter the lysosome for recycling, and much more.
Microtubules also form cilia and flagella that allow for cells to move larger extracellular particles or to move themselves. For example, ciliated cells form the lining of our respiratory tract and help move mucus, dust, and dirt away from our lungs. While sperm use their flagella to move towards an egg in hopes of fertilizing it.
Cell Transport and Trafficking
Now that we have briefly discussed microtubules and the motor proteins dynein and kinesin let’s focus on cell transport and trafficking in more detail.
Anterograde and Retrograde
While we are most familiar with anterograde transport or transport starting from the Golgi apparatus moving outwards to other parts of the cell, transport occurs in two directions. This means that materials can also be shuttled backward from the outside of the cell towards the inside of the cell in a process called retrograde transport.
While the path traveled by each type of transport is often the same the direction is reversed. For example in anterograde transport, proteins are synthesized in the RER transported to the Golgi apparatus and shuttled around or out of the cell in little membrane-bound packages called vesicles.
Whereas in retrograde transport a virus might be enveloped in a specialized membrane-bound vesicle called an endosome. While many endosomes often merge with lysosomes in order to destroy or recycle their contents others will transport endosomes back to the nucleus.
While transport itself is going on in two ways the motor proteins, kinesin and dynein, only travel in one direction. Here kinesin facilitates anterograde transport from the cell interior to the cell membrane while dynein carries out retrograde transport from the cell membrane inwards to the cell interior.
Routing
As we have seen different products are trafficked to different organelles or parts of the cell but how did the Golgi know where to send stuff? Molecular shipping labels!
These shipping labels take the form of a wide array of modifications including phosphorylation, glycosylation, ubiquitination, among others. While many different signaling molecules and modifications exist there are only three we need to know: signal sequences, nuclear localization sequences, and ubiquitin.
Signal sequences are incorporated into peptides and cause proteins to be synthesized into the RER this allows molecules to travel to the Golgi. There they will be packaged and are often sent secreted from the cell or incorporated into the cell membrane.
Nuclear localization sequences on the other hand are tags that instruct a product to be transported back to the nucleus. Often transcription factors carry these modifications and a nuclear localization sequence will allow them to enter the nucleus through nuclear pores so they can control gene expression.
Lastly, ubiquitin targets proteins for degradation in the proteasome. As the name suggests ubiquitin is ubiquitous and actually has a wide array of other roles within the cell thankfully we only need to know about its role in protein degradation for the MCAT.
