All organisms have a cell membranes to maintain homeostasis. However, in eukaryotic cells there is a more elaborate method of maintaining homeostasis and interacting with the environment. Eukaryotic cells have 3 types of proteins associated with the cell membrane to interact with the cell and its environment:
- Integral Membrane Protein
- Peripheral Protein
- Lipid Anchored Proteins
[gn_note color=”#ffcc00″]Quick Fact: More than half of proteins produced in cells interact with the membrane.[/gn_note]
Integral proteins are “integrated” into the cell membrane. These proteins are attached to the membrane for long durations. There are 2 types of integral proteins
- Transmembrane: they span the entire membrane (example: ion channels)
- Monotopic: Proteins that are only embedded into one side of the cell membrane.
Peripheral proteins are found on the perimeter our outside of the cell membrane. Peripheral proteins do not penetrate the cell membrane itself. An example of a peripheral proteins is Cytochrome C found in the electron transport chain. Cytochrome c participates in passing electrons but does not penetrate the inner membrane of the mitochondria.
Lipid Anchored Proteins
Lipid anchored proteins, also known as Glycosylphosphatidylinositol (GPI) anchors, are covalently attached to the cell membrane rather that connected through a hydrogen bond like peripheral proteins. GPI anchors are found in all cell types and are associated with lipid rafts. The GPI anchors may be involved in protein/lipid sorting, and signaling transduction.