Like a strand of white pearls, DNA wraps around an assembly of special proteins called histones (colored) to form the nucleosome, a structure responsible for regulating genes and condensing DNA strands to fit into the cell's nucleus. (NIGMS) Image credit: Karolin Luger, Colorado State University.

Proteins: molecular chains built up from twenty different building blocks, the amino acids. (Watson, 36) Most abundant molecule in the body. (Norton Lectures, 6/17/09) A large molecule held together in a complex three-dimensional structure. Proteins play regulatory, structural, and catalytic roles in living systems. (Kandel, 446)

Each “cell type” in the human body contains a unique mix of proteins - a sub-population of all the proteins available to the cell. This mix of proteins enables the cell to perform its specific biological functions. (Kandel, 257) DNA “codes” for proteins. Hundreds of proteins called “enzymes” are needed in all biochemical reactions. (Norman Labs, 81) Used in virtually all of the body’s functions - from building cells to manufacturing “hormones” to regulating brain activity. The brain requires hundreds of thousands of different proteins to function. (Hockenbury 353) Most proteins are "degraded" (converted into simpler substances – chemically broken down) and destroyed in a period of hours. (Kandel, 272) They have a lifetime that ranges from a matter of seconds to days. They have to endure the 'tumult' within a cell, where heat energy sends molecules ricocheting around. At any given moment, a human cell typically contains thousands of different proteins, with some being manufactured and others being discarded. (Venter, 41) The lifespan of every protein in the cell is preprogrammed in the "genetic code." All cells will die if they cannot make new proteins on a continuous basis to replace those that are damaged or misfolded. (Venter, 43) Protein functions include anchoring “cytoskeleton” and “extracellular” substances, catalyzing "chemical reactions," acting as "receptor" sites, "carrier" molecules, and ("channel proteins") in membranes, and identity marking for cellular self-recognition and tissue recognition, (Norton Lectures, 6/3/09) Some proteins travel to and function in the nucleus, such as "transcription factors." (Lewis, 191)


Chaperone Proteins: mediate the correct assembly or disassembly of "polypeptides" and their associated "ligands." Although they take part in the assembly process, molecular chaperones are not components of the final structures. (MeSH) Aid protein-folding and block the formation of harmful aggregates, as well as dismantle aggregates that do form. (Venter, 42) Diverse family of intracellular proteins involved in the assembly and "translocation" of other proteins; seem to function by stabilizing partially unfolded states; includes certain heat shock proteins. (GHR) Stabilize partially folded regions in their correct form, and prevent a protein from getting 'stuck' in an intermediate form. (Lewis, 191) Also referred to as 'molecular chaperones.'

Drystrophin: a protein (found) in “muscles.” (PubMedHealth) Encoded by the human ‘DMD gene.’ This protein is involved in anchoring the “cytoskeleton” to the “plasma membrane” in muscle cells. (NCIt)

Glycoprotein: a protein that has small carbohydrate attached to it. (Brooker, G-16) A protein that has sugar molecules attached to it. (NCIt)

Fibronectin: a “glycoprotein” found on the surfaces of cells. The proteins are lost or reduced when these cells undergo viral or chemical transformation. (MeSH) Connects cells to the extracellular matrix and helps to organize components in the "ECM." (Booker, 192)

Laminin: connects cells to the extracellular matrix and helps to organize components in the ‘basal lamina,’ a specialized ECM found next to “epithelial cells.” (Booker, 192) Major glycoprotein of 'basement membrane' and other extracellular matrix structures; serves as an adhesive … and cell activator during “neurogenesis” and other cell growth events. (NCIt)

Membrane Proteins: proteins that bind to cellular membranes. (Brooker, 57) Embedded in the (membrane) bilayer, some traverse the entire structure, while others extend from one or both faces. (Lewis, 26) Participate in some of the most important and interesting cellular processes. These include transport, energy "transduction," cell "signaling," "secretion," cell recognition, and cell-to-cell contact. Approximately 25% of all “genes” “encode” membrane proteins. Approximately 70% of all medications exert their effects by binding to membrane proteins. (Brooker, 87) Depending on the cell type, membrane proteins may be restricted in their movement. Membrane proteins may be attached to molecules that are outside the cell. (Brooker, 90)

Adherens Junctions: connect cells to each other via “cadherins.” In the “cytosol,” adherens junctions bind to “actin filaments.” (Brooker, 196) Cell to cell adherens junctions, the most common type of intercellular adhesions, are important for maintaining tissue architecture and cell polarity and can limit cell movement and proliferation. (NCIt)

Anchoring Junctions: types of junctions that occur between cells and within the “extracellular matrix.” Cell junctions that hold adjacent cells together or bond cells to the extracellular matrix. Mechanically strong. Rely on the function of “cell adhesion molecules." (Brooker, 196)

Tight Junctions: partial fusion of membranes forming tight, leak proof barriers between cells. (Norman, 6/10/09) Junctions between adjacent cells that prevent the leakage of material between cell layers. (Brooker, 196)

Cell Adhesion Molecules (CAMs): membrane proteins incorporated in anchoring junctions. Proteins which bind membranes of adjacent cells forming cell sheets. (Norman, 6/10/09) Usually glycoproteins. Mediate cell-to-cell adhesion. Their functions include the assembly and interconnection of various “vertebrate” systems, as well as maintenance of tissue integration, wound healing, movements, cellular migrations, and “metastasis.” (MeSH) Have multiple binding sites that “bind” to other components in the ECM such as protein fibers and carbohydrates. These same proteins also have binding sites for receptors on the surfaces of cells. (Brooker, 193) Help guide "white blood cells" to an injured area (as part of the “immune response.”) Help guide cells surrounding an “embryo” to grow toward maternal cells and form the “placenta.” (Lewis, 36)

Cadherins: calcium-dependent cell adhesion proteins. (MeSH) Cell adhesion molecules involved in cell-to-cell junctions. Each cadherin has two identical "subunits." The extracellular domains of the two cadherin subunits, each in adjacent cells, bind to each other to promote cell-to-cell adhesion. Having different types of cadherins allows different types of cells to recognize each other. (Brooker, 196) 

Desmosomes: a type of junction that attaches one cell to its neighbor. (MeSH) Connect cells to each other via cadherins. They are spot-like points of intercellular contact that rivet cells together. In the cytosol, connected to "intermediate filaments." (Brooker, 196) Desmosomes assemble in response to cell to cell contact and raised levels of extracellular calcium. Sensitivity to calcium levels is lost as desmosomes mature. (NCIt)

Focal Adhesions: a cellular junction where cellular transmembrane "integrin" receptors bind to extracellular matrix proteins. (NCIt) In the cytosol, focal adhesions bind to actin filaments. (Brooker, 197)

Integrins: a type of cell adhesion molecule that creates connections between cells and the extracellular matrix. A group of cell-surface "receptor" proteins. Like cadherins, integrins bind to actin or intermediate filaments in the cytosol of the cell. Abnormalities in cell adhesion molecules, such as integrins, are often associated with the ability of cancer cells to “metastasize.” (Brooker, 198-199) Latch onto white blood cells (and help) guide them to any injury site. (Lewis, 36)

Selectins: family of structurally related cell surface proteins. They mediate weak adhesion between (white blood cells) and endothelial cells during "immune response" and "inflammation." (Lawrence) Attach to white blood cells and slow them to a roll by also binding to carbohydrates on the capillary wall. (Lewis, 36)

Integral Membrane Proteins: capable of rotational and lateral movement. (Brooker, 90) Cannot be released from the membrane. (Brooker, 87) Also referred to as ‘extrinsic membrane proteins.’

Peripheral Membrane Proteins: bound to regions of integral membrane proteins that project out from the membrane, or they are bound to the “polar head” groups of “phospholipids.” (Brooker, 87)

Mitochondrial Proteins: proteins encoded by the mitochondrial genome or proteins encoded by the nuclear genome that are imported to and resident in the mitochondria. (MeSH)

Motor Proteins: a category of cellular proteins that uses “ATP” as a source of energy to promote movement. (Brooker, 71) Consists of a 'head' used as an ATP “binding” site, a 'hinge' which functions as the point of movement, and a 'tail' which functions as a binding site for other molecules (for example a track of “microtubules.”) Movements include 'cargo' movement from one point to another using motor protein ‘legs’ to ‘walk’ the molecule to a destination, for example -  transport “vesicles.” Includes movement of another molecule where the motor protein is stable, for example, “muscle” movement. Includes displacement of another molecule, for example, “flagella” or “cilia” movement. (Norman, 6/2/09)

Neuroligin Protein: a neuronal cell surface protein. (MeSH) Cell adhesion protein embedded in the neuron membrane. Strengthens synaptic connections in “postsynaptic” “glutamate” neurons associated with “learning” and “memory.” Mutations that cause misfolding may impair communication in a way that causes “autism.” (Lewis, 159)

Proteasome: responsible for the elimination of abnormal proteins via a peptide-bond-breaking reaction carried out by enzymes called "proteases." (Venter, 43) A tunnel-like multi-protein structure. As a protein moves through the opening, it is stretched out, chopped up, and its peptide pieces degraded into amino acids. Proteasomes also destroy properly folded proteins that are in excess or no longer needed. (Lewis, 192)

Proteoglycan: a protein that has a large carbohydrate attached to it. (Norman, 6/17/09) Glycoproteins which have a very high “polysaccharide” content. (MeSH) Proteoglycans are found in "cartilage" and other "connective tissues." (NCIt)

Protein Isoforms: different forms of a protein, that may be produced from different genes, or from the same gene by "alternative splicing." (MeSH) The protein products of different versions of messenger RNA created from the same gene by employing different promoters, which causes transcription to skip certain "exons." Since the promoters are tissue-specific, different tissues express different protein products of the same gene. (GeneReviews) Different combinations of exons of a gene encode different versions of a... protein. (Lewis, 185) Also referred to as 'isoforms.'

Structural Proteins: proteins whose major function is to give shape and support to tissues, cells, and subcellular structures. (NCIt) Form large fibers that give the extracellular matrix its strength and elasticity. (Brooker, 192)

Collagen: a protein that is the principal constituent of white fibrous connective tissue. Also found in skin, bone, cartilage, and “ligaments.” It is relatively inelastic but has a high 'tensile strength.' (OxfordMed) Forms large fibers and interconnected fibrous networks in the extracellular matrix. (Booker, 193) Comprises about one third of the total protein in “mammalian” organisms. (MeSH)

Elastin: a protein that makes up “elastic” fibers in the extracellular matrix of animals. (Brooker, G-12) This protein is involved in the elasticity of the extracellular matrix. (NCIt)