Membrane Compartments Of The Cell

Membrane compartments of the cell Cytoskeleton

Svetlana Lutsenko, Dept. Biochemistry and Mol. Bology MRB 624, Ext. 4-6953, [email protected]

Reading for the “Cytoskeleton” portion: Alberts “Essential Cell Biology” pp.513-532

Mechanisms by which molecules can pass through membranes 1.

Transport of various molecules, including proteins, DNA and RNA through large protein-lined pores. Specificity of transport is based on size exclusion

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Simple diffusion and proteinmediated transport of ions and small molecular weight nutrients, such an amino-acids and sugars

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Vesicular transport is mediated by membranes that bud off as a vesicles from their original compartment and carry out specific cargo (ions, proteins, neurotransmitters) to another compartment

Cellular Compartments and Their Functions •

Cytosol : many biosynthetic pathway; synthesis of cytosolic protein

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Nucleus : DNA and RNA synthesis Mitochondria : energy metabolism, urea cycle

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Endoplasmic Reticulum: Protein Secretion; Synthesis of Membrane Proteins

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Golgi : Distribution and Modification of Secreted Proteins

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Lysosomes : Degradation of proteins and other molecules

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Peroxisomes : Degradation of Certain Fatty Acid

Nucleus • The nucleus is a membrane bound structure that contains the cell's hereditary information and controls the cell's growth and reproduction. • It is commonly the most prominent organelle in the http://biology.about.com/library/weekly/aa032300a.htm cell

Nuclear Membranes •

The nuclear content is located in the nuclear lumen and is surrounded by a double membrane or nuclear envelope, envelope composed of inner membrane and outer membrane. membrane

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The outer membrane is contiguous with the ER

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The nuclear membrane contains nuclear pores, pores which provide selective access into and out of the nuclear lumen

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The inner membrane has a protein lining called the nuclear lamina, which binds to chromatin and other nuclear components.

The Nuclear Pore Complexes form a continuous aqueous channel between cytoplasm & nucleoplasm nuclear pore complex is large –about 120 million Daltons

-30 different proteins found in pore; basic subunit repeated 16 times. “Central granule" now called "transporter" functions to move molecules through pore. There is a ring of proteins that anchors pore to N.E. and the "basket" of fibers with unknown function -no obvious motor proteins were found

Proposed mechanism -transport factors with cargo destined to pass through nuclear pore bind to pore increasing their local concentration. Higher local concentration allows diffusion across the diffusion barrier channel

The Nucleolus •

The region of the nucleus where portions of chromosomes that contain genes coding for ribosomal RNA are transcribed and ribosomal subunits are assembled •Stretch of DNA with rRNA genes nucleolar organizing region = (NOR) • Ribosomal proteins are synthesized in cytoplasm and transported into the nucleus

http://biology.about.com/library/weekly/aa032300a.htm

•These proteins self associate with appropriate rRNA during rRNA synthesis forming immature ribosomal subunits •Ribosomes finish self assembly in cytoplasm

Endoplasmic Reticulum

The Endoplasmin Reticulum (ER) is an extensive, extra-nuclear membrane system with the following functions: •

ER is a home for various enzymes involved in protein folding , drug detoxification, membrane lipid biosynthesis, cholesterol and fatty acid metabolism

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ER is an entry point for protein sorting. Targeting of these proteins is mediated by signal sequence. The membrane proteins are inserted into the ER membrane in their proper orientation. Secreted proteins are translocated into ER lumen and then transported to the destination place

Rough and Smooth ER •

Two regions of the ER differ in both structure and function. Rough ER has ribosomes attached to the cytoplasmic side of the membrane. Smooth ER lacks attached ribosomes. Typically, the smooth ER is a tubule network and the rough ER is a series of flattened sacs.

The smooth ER has a wide range of functions including carbohydrate and lipid synthesis. It serves as a transitional area for vesicles that transport ER products to various destinations.

http://esg-www.mit.edu:8001/esgbio/cb/org/er.gif

In liver cells the smooth ER produces enzymes that help to detoxify certain compounds. In muscles the smooth ER assists in the contraction of muscle cells and in brain cells it synthesizes male and female hormones.

The rough ER manufactures membranes and secretory proteins. In leukocytes the rough ER produces antibodies. In pancreatic cells the rough ER produces insulin.

The rough and smooth ER are usually interconnected and the proteins and membranes made by the rough ER move into the smooth ER to be transferred to other locations.

The cytoplasm has a reducing environment, while ER lumen is oxidizing . This difference is generated by unequal distribution of trypeptide glutathione and is essential for formation of disulfide bonds in proteins and for proper folding

Mitochondria

Mitochondria (singular: mitochondrion) are the sites of aerobic respiration, and generally are the major energy production center in eukaryotes

The number of mitochondria range from one to thousands per cells. They are often positioned in cells nearest to sites of energy utilization One of the richest sources of mitochondria is a hummingbird flight muscle

Mitochondria are a double membrane organelle in which the inner membrane is in-folded to form “cristae”. The outer membrane is a fairly simple phospholipid bilayer, containing porins, proteins that render it permeable to molecules of about 10 kilodaltons or less. Ions, nutrient molecules, ATP, ADP, etc. easily pass through the outer membrane and enter the intermembrane space The inner membrane is more complex and contains respiratory chains and transporters The matrix lies within the inner membrane. The access to this compartment often requires specific transporters

Four possible localization for mitochondrial enzymes

The Golgi Complex

Located near cell nucleus, consists of flattened, membrane-bounded sacs (cisternae) forming a stack Each stack has: cis-face is an entry face - adjacent to ER to accept incoming vesicles trans-face is an exit face – points towards plasma membrane, produces vesicles for forward flow

The function of the Golgi is to transport and process secreted and membrane proteins from ER to the cell surface Cisternae segregated into convex ("cis"), medial (middle), and concave ("trans") compartments. • • ER

• cys medial trans TGN

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Cis – removal of mannose, phosphorylation Medial – removal of mannose, addition of N-acetylglucosamine Trans – Removal of galactose, addition of sialic acid TGN – addition of sialic acid, Sorting

Vesicular Transport

The Endocytic Pathway Endosomes and Lysosomes

Lysosomes •Lysosomes are active in recycling the cell's organic material and in the intracellular digestion of macromolecules. •Lysosomes contain various hydrolytic enzymes that are capable of digesting nucleic acids, polysaccharides, fats and proteins. •The inside of a lysosome is acidic. •In humans, a variety of inherited conditions can affect lysosomes. These defects are called storage diseases and include Pompe's disease and Tay-Sachs disease. People with these disorders are missing one or more of the lysosomal hydrolytic enzymes.

The Cytoskeleton

Cytoskeleton is a network of protein filaments in the cytoplasm Main functions: • Supports large volume of the cytoplasm • Participates in large-scale movements associated with the changes in cell shape and cell motility • Provides machinery for organelle transport, chromasome segregation during mitosis, and cell division

Major components of cytoskeleton Microtubules

Actin filaments

Intermediate filaments

The cytoskeletal filaments • Common Features : – Linear polymers of protein subunits • Actin ( ~8 nm in diameter) • Intermediate Filaments ( ~10 nm in diameter) • Microtubules ( ~24 nm in diameter)

– Filaments are dynamic, i.e. they can assemble and disassemble – Highly conserved

Intermediate Filaments • Intermediate filaments enable cells to withstand mechanical stress when cells are stretched. • They can span the entire cytoplasm and are anchored to the plasma membrane.

The Microtubule Cytoskeleton •

Also penetrates the entire volume of the cell

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Whereas actin fibers are concentrated at the periphery, most microtubules radiate from a central location in the cell

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Main functions: intracellular transport and mitosis

Microtubules Microtubules provide an organizational structure in an interphase cell and separate chromosomes in a dividing cell

http://www.circs.neu.edu/external/Frank.Gibbons/spindle.html

Microtubules Provide Tracks for Transport

Microtubules are long hollow cylinders made of tubulin

•Protofilaments are linear chains of tubulin dimers, a parallel bundle of 13 protofilaments forms a microtubule •There are three kinds of tubulins, each with many subtypes: α-tubulin and β-tubulin form α/β tubulin dimers and represent the basic building block of microtubules γ-tubulin is involved in more specialized processes, such as nucleation Microtubules have a GTP “cap” stabilizing the ends.

Motor Proteins • Motor proteins bind to microtubules and move by cycles of conformational changes using energy from ATP. • One end of the protein can bind to specific cellular components.

Actin filaments = microfilaments •Actin is the most common protein in the cytoplasm •Actin filaments are concentrated beneath the plasma membrane and give the cell mechanical strength •Assembly of actin filaments can determine cell shape and cause cell movement •Association of actin filaments with myosin can form contractile structures

Spectrin Principal component of the cytoskelton (protein meshwork underlying surface of the red cell) –Maintains structural integrity of the red cell (e.g. biconvave shape) –Long thin flexible rod –Necessary as red cells go through small capillaries