membrane protein

Examples of membrane protein in the following topics:

• Fluid Mosaic Model

  • The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary with cell type.
  • The mitochondrial inner membrane contains 76% protein and 24% lipid.
  • Proteins make up the second major component of plasma membranes.
  • Some complex proteins are composed of up to 12 segments of a single protein, which are extensively folded and embedded in the membrane.
  • Integral membrane proteins may have one or more alpha-helices that span the membrane (examples 1 and 2), or they may have beta-sheets that span the membrane (example 3).

• Facilitated transport

  • Facilitated diffusion is a process by which molecules are transported across the plasma membrane with the help of membrane proteins.
  • Unlike simple diffusion where materials pass through a membrane without the help of proteins, in facilitated transport, also called facilitated diffusion, materials diffuse across the plasma membrane with the help of membrane proteins.
  • Another type of protein embedded in the plasma membrane is a carrier protein.
  • They may cross the plasma membrane with the aid of channel proteins.
  • Carrier proteins change shape as they move molecules across the membrane.

• Exocytosis

  • In exocytosis, waste material is enveloped in a membrane and fuses with the interior of the plasma membrane.
  • Next, the vesicle's membrane and the cell membrane connect and are held together in the vesicle docking step.
  • This stage of exocytosis is then followed by vesicle priming, which includes all of the molecular rearrangements and protein and lipid modifications that take place after initial docking.
  • The final stage, vesicle fusion, involves the merging of the vesicle membrane with the target membrane.
  • Some examples of cells using exocytosis include: the secretion of proteins like enzymes, peptide hormones and antibodies from different cells, the flipping of the plasma membrane, the placement of integral membrane proteins(IMPs) or proteins that are attached biologically to the cell, and the recycling of plasma membrane bound receptors(molecules on the cell membrane that intercept signals).

• Selective Permeability

  • In fact, there is a considerable difference between the array of phospholipids and proteins between the two leaflets that form a membrane .
  • On the interior of the membrane, some proteins serve to anchor the membrane to fibers of the cytoskeleton.
  • There are peripheral proteins on the exterior of the membrane that bind elements of the extracellular matrix.
  • Carbohydrates, attached to lipids or proteins, are also found on the exterior surface of the plasma membrane.
  • Simple sugars and amino acids also need help with transport across plasma membranes, achieved by various transmembrane proteins (channels).

• The Endoplasmic Reticulum

  • The endoplasmic reticulum (ER) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids.
  • The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.
  • These modified proteins will be incorporated into cellular membranes—the membrane of the ER or those of other organelles—or secreted from the cell (such as protein hormones, enzymes).
  • The RER also makes phospholipids for cellular membranes.
  • If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER's membrane.

• Components of Plasma Membranes

  • The plasma membrane (also known as the cell membrane or cytoplasmic membrane) is a biological membrane that separates the interior of a cell from its outside environment.
  • Composed of a phospholipid bilayer with embedded proteins, the plasma membrane is selectively permeable to ions and organic molecules and regulates the movement of substances in and out of cells.
  • Among the most sophisticated functions of the plasma membrane is its ability to transmit signals via complex proteins.
  • Marker proteins on human red blood cells, for example, determine blood type (A, B, AB, or O).
  • The plasma membrane is composed of phospholipids and proteins that provide a barrier between the external environment and the cell, regulate the transportation of molecules across the membrane, and communicate with other cells via protein receptors.

• Plasma Membrane Hormone Receptors

  • Lipid-insoluble hormones bind to receptors on the outer surface of the plasma membrane, via plasma membrane hormone receptors.
  • When a hormone binds to its membrane receptor, a G protein that is associated with the receptor is activated.
  • G proteins are proteins separate from receptors that are found in the cell membrane.
  • The activated G protein in turn activates a membrane-bound enzyme called adenylyl cyclase.
  • Cellular responses to hormone binding of a receptor include altering membrane permeability and metabolic pathways, stimulating synthesis of proteins and enzymes, and activating hormone release.

• Membrane Fluidity

  • First, the mosaic characteristic of the membrane helps the plasma membrane remain fluid.
  • The integral proteins and lipids exist in the membrane as separate but loosely-attached molecules.
  • Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts.
  • The plasma membrane is a fluid combination of phospholipids, cholesterol, and proteins.
  • Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane.

• The Plasma Membrane and the Cytoplasm

  • Despite differences in structure and function, all living cells in multicellular organisms have a surrounding plasma membrane (also known as the cell membrane).
  • The plasma membrane can be described as a phospholipid bilayer with embedded proteins that controls the passage of organic molecules, ions, water, and oxygen into and out of the cell.
  • Active transport may take place with the help of protein pumps or through the use of vesicles.
  • The cytoplasm is the location for most cellular processes, including metabolism, protein folding, and internal transportation.
  • The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it.

• Regulating Protein Activity and Longevity

  • Sometimes these modifications can regulate where a protein is found in the cell; for example, in the nucleus, the cytoplasm, or attached to the plasma membrane.
  • The addition of methyl groups to a protein can result in protein-protein interactions that allows for transcriptional regulation, response to stress, protein repair, nuclear transport, and even differentiation processes.
  • Methylation in the proteins negates the negative charge on it and increases the hydrophobicity of the protein.
  • The addition of an ubiquitin group to a protein marks that protein for degradation.
  • These proteins are moved to the proteasome, an organelle that functions to remove proteins to be degraded .