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Glycoproteins: Conjugated protein-carbohydrate compounds including mucins, mucoid, and amyloid glycoproteins.

Determination of Molecular Structures of HIV Envelope Glycoproteins using Cryo-Electron Tomography and Automated Sub-tomogram Averaging

1Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, 2The Medical Research Council Mitochondrial Biology Unit, University of Cambridge, 3National Library of Medicine, National Institutes of Health, 4Massachusetts Institute of Technology, 5William Fremd High School, 6University of Virginia, 7Duke University, 8Yale University, 9University of Notre Dame, 10Washington University in St. Louis, 11Bioinformatics and Computational Biosciences Branch, National Institute of Allergy and Infectious Diseases, National Institutes of Health, 12Thomas Jefferson High School for Science and Technology

JoVE 2770

 Immunology and Infection

Protein Associations

JoVE 10704

The cell membrane—or plasma membrane—is an ever-changing landscape. It is described as a fluid mosaic as various macromolecules are embedded in the phospholipid bilayer. Among the macromolecules are proteins. The protein content varies across cell types. For example, mitochondrial inner membranes contain ~76%, while myelin contains ~18% protein content. Individual cells contain many types ofbrane proteins—red blood cells contain over 50—and different cell types harbor distinct membrane protein sets. Membrane proteins have wide-ranging functions. For example, they can be channels or carriers that transport substances, enzymes with metabolic roles, or receptors that bind to chemical messengers. Like membrane lipids, most membrane proteins contain hydrophilic (water-loving) and hydrophobic (water-fearing) regions. The hydrophilic areas are exposed to water-containing solution inside the cell, outside the cell, or both. The hydrophobic regions face the hydrophobic tails of phospholipids within the membrane bilayer. Membrane proteins can be classified by whether they are embedded (integral) or associated with the cell membrane (peripheral). Most integral proteins are transmembrane proteins, which traverse both phospholipid layers, spanning the entire membrane. Their hydrophilic regions extend from both sides of the membrane, facing cytosol on

 Core: Biology

What are Membranes?

JoVE 10971

A key characteristic of life is the ability to separate the external environment from the internal space. To do this, cells have evolved semi-permeable membranes that regulate the passage of biological molecules. Additionally, the cell membrane defines a cell’s shape and interactions with the external environment. Eukaryotic cell membranes also serve to compartmentalize the internal space into organelles, including the endomembrane structures of the nucleus, endoplasmic reticulum and Golgi apparatus. Membranes are primarily composed of phospholipids composed of hydrophilic heads and two hydrophobic tails. These phospholipids self-assemble into bilayers, with tails oriented toward the center of the membrane and heads positioned outward. This arrangement allows polar molecules to interact with the heads of the phospholipids both inside and outside of the membrane but prevents them from moving through the hydrophobic core of the membrane. Proteins and carbohydrates contribute to the unique properties of a cell’s membrane. Integral proteins are embedded in the membrane, while peripheral proteins are attached to either the internal or external surface of the membrane. Transmembrane proteins are integral proteins that span the entire cell membrane. Transmembrane receptor proteins are important for communicating messages from the outside to the ins

 Core: Biology

The Fluid Mosaic Model

JoVE 10698

The fluid mosaic model was first proposed as a visual representation of research observations. The model comprises the composition and dynamics of membranes and serves as a foundation for future membrane-related studies. The model depicts the structure of the plasma membrane with a variety of components, which include phospholipids, proteins, and carbohydrates. These integral molecules are loosely bound, defining the cell’s border and providing fluidity for optimal function. The most abundant component of the fluid mosaic model is lipids. Lipids include both phospholipids and cholesterols. Phospholipids are amphipathic, having both hydrophobic and hydrophilic parts. They consist of a hydrophilic—water-loving—head, and two hydrophobic—water-fearing—fatty acid tails. Phospholipids spontaneously form a lipid bilayer that separates the inside of the cell from the outside. The lipid bilayer consists of the hydrophobic tails facing inward and the hydrophilic heads facing the aqueous environment inside and outside the cell. Cholesterols are a class of steroids that play a role in regulating membrane fluidity and flexibility. Membrane fluidity facilitates the transport of specific molecules and ions across the plasma membrane. The second major component of the mosaic is proteins. Proteins can differentially associate with the li

 Core: Biology

Golgi Apparatus

JoVE 10970

As they leave the Endoplasmic Reticulum (ER), properly folded and assembled proteins are selectively packaged into vesicles. These vesicles are transported by microtubule-based motor proteins and fuse together to form vesicular tubular clusters, subsequently arriving at the Golgi apparatus, a eukaryotic endomembrane organelle that often has a distinctive ribbon-like appearance.

The Golgi apparatus is a major sorting and dispatch station for the products of the ER. Newly arriving vesicles enter the cis face of the Golgi—the side facing the ER—and are transported through a collection of pancake-shaped, membrane-enclosed cisternae. Each cisterna contains unique compositions of enzymes and performs specific protein modifications. As proteins progress through the cis Golgi network, some are phosphorylated and undergo removal of certain carbohydrate modifications that were added in the ER. Proteins then move through the medial cisterna, where they may be glycosylated to form glycoproteins. After modification in the trans cisterna, proteins are given tags that define their cellular destination. Depending on the molecular tags, proteins are packaged into vesicles and trafficked to particular cellular locations, including the lysosome and plasma membrane. Specific markers on the membranes of these vesicles allow them to dock

 Core: Biology

The Extracellular Matrix

JoVE 10695

In order to maintain tissue organization, many animal cells are surrounded by structural molecules that make up the extracellular matrix (ECM). Together, the molecules in the ECM maintain the structural integrity of tissue as well as the remarkable specific properties of certain tissues.

The extracellular matrix (ECM) is commonly composed of ground substance, a gel-like fluid, fibrous components, and many structurally and functionally diverse molecules. These molecules include polysaccharides called glycosaminoglycans (GAGs). GAGs occupy most of the extracellular space and often take up a large volume relative to their mass. This results in a matrix that can withstand tremendous forces of compression. Most GAGs are linked to proteins—creating proteoglycans. These molecules retain sodium ions based on their positive charge and therefore attract water, which keeps the ECM hydrated. The ECM also contains rigid fibers such as collagens—the primary protein component of the ECM. Collagens are the most abundant proteins in animals, making up 25% of protein by mass. A large diversity of collagens with structural similarities provide tensile strength to many tissues. Notably, tissue like skin, blood vessels, and lungs need to be both strong and stretchy to perform their physiological role. A protein called elastin gives p

 Core: Biology
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