PHYSIO Cell & membrane fnxn

Where are secretory & membrane proteins synthesized?

in the rough ER (RER)

Lipid membrane permeability depends on

lipid:aqueous partition coefficient

Both cilia and flagella are composed largely of

microtubules

What does stability of microtubules depend on?

rate of polymerization (addition of tubulin subunits) to the rate of GTP hydrolysis to GDP by beta-tubulin

How does membrane maintain concentration gradient?

- Bilayer has TM proteins that are active pumps (i.e. Na/K pump) - Concentration gradient is potential energy - No ATP --> No Pump --> no concentration gradient --> cell death

How is vesicular trafficking regulateD?

- Binding to small G-proteins (GTP/GDP binding proteins) that regulate assembly & delivery of contents - SNARE proteins: 2 types; vesicular (vSNAREs) on vesicular membranes interact with target (tSNAREs) on cell membrane; ---- e.g., fusion of vesicles in terminal of cholinergic or adrenergic neurons result in EXOCYTOTIC release of neurotransmitter - Vesicles contain STRUCTURAL PROTEINS or lipids in their membranes to direct specific membrane compartments to the right place; --- vSNARE on vesicle interacts w/ tSNARE on target cell membrane --> fusion of vesicles --> exocytosis of NT

Constitutive vs Regulated secretion in secretory proteins:

- Constitutively-secreted proteins are made constantly and secreted immediately --- e.g., leukocytes, hepatocytes, keratinocytes, fibroblasts - Regulated-proteins are made on demand and secreted upon signal transduction --- Ca or cAMP are examples of signals

Microtubules form the framework/highway along which proteins are transported; what are 2 major motor proteins?

- Dyneins move things RETROgrade; D for Disco; disco is old - Kinesins move things ANTEgrade --- both use energy derived from hydrolysis of ATP to move chromosomes, vesicles & other organelles along microtubules --- both unidirectional

Main fnxns of intermediate filaments:

- Neurofilaments are found in nerve cells & help stabilize projections from cell body. - In skeletal muscle intermediate filaments (IFs) stabilize actin-myosin interaction. - Skin strength due partly to interlocking IF's, without IF's cells would rupture & skin would blister - can be used as cellular markers --- e.g. vimentin is found in fibroblasts ----- epithelial cells have cytokeratin

What type of receptors are Nicotonic? What happens when they activate?

- Nictonic receptors are Ligand- gated Na+ channel - opens in response to ACh binding to alpha- subunits on the receptors - Na+ moves down a concentration gradient --> forms ion channel

Adhesion molecules

- Type of integral membrane protein - have integrins (Cell-matrix adhesion molecules) that allow communication b/w a cell & its surrounding tissue (adaptation). - Critical for cell development, growth, cellular shape & mobility. - Cadherins= Ca2+ dependent cell adhesion molecules; -- e.g., glycoprotein - N-CAMs= Ca2+ independent neural cell adhesion molecules that fnxn in the migration of immune cells & development of axons ** Loss of cell adhesion molecules is observed in metastatic tumors

How do voltage gated channels open and close? What does the transmembrane protein have?

- Voltage gated (activated) Na+ channels open & close as the membrane is depolarized or hyperpolarized respectively. -The transmembrane protein has voltage sensors & a selectivity filter

Tubulin is

- a heterodimer of alpha & beta subunits stacked with ~ 13 units/ring - alpha subunit = (-) end - Beta subunit = (+) end - alpha & beta subunits both bind GTP but only beta subunit has GTPase activity - Tubulin is added to either end but addition is 2x as rapid at the (+) end

Ankyrin

- a peripheral membrane protein that "anchors" the cytoskeleton of RBCs to an integral membrane transport protein

How do hydrophilic substances move into the cell?

- alpha-helices can line up in a way to facilitate movement of hydrophilic molecules or ions across the membrane (helix will have hydrophilic surface along some edges and hydrophobic surfaces along others) - pumps, carriers and pores form a pathway bc alpha-helices along the TM domain are AMPHIPHATIC --> ions pass through these pores

Cholesterol

- another lipid of plasma membrane -S TEROID ring binds to fatty- acid side chains partially immobilize them. - cholesterol provides rigidity to plasma membrane --> stiffens it --- Hypercholesteremia disrupts the ability of PLs to interact normally with each other

Smooth ER is abundant in

- cells involved in lipid metabolism & synthesis; -- e.g., steroid hormones (adrenal cortical cells, ovary, testes)

What are products in golgi that are destined to stay in the cell membrane coated with?

- clathrin protein covering - clathrin forms a lattice around the vesicle & associated proteins that later serve as recognition signal ---- e.g., receptor mediated endocytosis (clarithin coated pits)

Microfilaments important for:

- contractile systems, such as muscle - cells moving away from each other as they divide during mitosis - ameboid movement in eukaryotic cells --- e.g. white blood cells, fibroblasts etc. move by putting forth pseudopodia

Examples of integral membrane proteins

- enzymes/pumps - intestinal lumen cells have epithelial cells w/ IMPs that fnxn in absorption - GPI-linked class (glycophosphatidylinositol): may function in 2nd messenger systems Major categories: 1. ligand binding receptors 2. ion channels/pores 3. adhesion molecules

Peripheral membrane proteins are

- hydrophilic - can be removed by salt solutions that disrupt ionic/hydrogen bonds

Microtubules important for:

- involved w/ intracellular protein trafficking - Transport of secretory vesicles; especially in neurons where NTs made in the cell body are transported to an axon terminal that may be very far away. - Sperm movement - cell division (mitosis)

Glycocalyx

- peripheral membrane protein consisting of negatively charged glycosaminoglycans; - it gives plasma membrane a fuzz-like coating - Due to their NEGATIVE charge, they repel NEGATIVELY charged substances - can attach to other cells & are involved in cell-cell recognition - May be a part of the immune response ** Play a major role in renal corpuscle where it is found on the lining of capillary endothelium; the negative charge prevents (repells) large negatively charged macromolecules from leaking through the glomeruli.

Main functions of phospholipid within the cell

1. Forming hydrophobic barrier (compartmentalizes molecules to functionally distinct locations in the cell) 2. creation of ion concentration gradient 3. important substrates for phospholipases

Functions of integral proteins

1. Ligand binding receptors 2. Ion channels/pores 3. adhesion molecules 4. carrier proteins 5. active pumps

2 types of Ion channels/ pores

1. Non-gated ion channels --- Leaky channels are always OPEN & ready for passive flow of ions down an electrochemical gradient 2. gated ion channels --- regulated by various stimuli; e.g., chemical (ligand), voltage changes, mechanical (stretch), light (photons)

3 types of integral proteins:

1. Transmembrane - Span lipid bilayer & is in contact w/ both ECF and ICF *** have hydrophobic alpha-helices miscible with hydrophobic bilayer 2. Embedded - do not span the membrane 3. Lipid anchors - protein bound to lipid by covalent bond ** e.g., glycophosphospholipid-linked proteins

____ of plasma membrane is made up of proteins

50%

How does cell get energy for transport?

Enzymes/Pumps catalyze ATP hydrolysis --> provides energy for transport against concentration gradient -- I.e Na/K ATPase, Ca2+ ATPase, SERCA pump, H+ pump

_____ stabilizes microtubules

GTP cap stabilizes microtubule; -- As GTP cap is hydrolyzed, tubulin is lost and the microtubule depolymerizes/shrinks

Major class of membrane lipids?

Glycerol based phospholipids Glycerol is esterified to fatty acids and phosphate (phosphate diester)

Secretory protein vs integral protein synthesis and transport:

Integral proteins: Synthesized in RER → post-translational modification in the Golgi → Covered with Clathrin → undergo sorting in the Trans-Golgi Network for vesicular trafficking to be inserted into the plasma membrane Secretory proteins: synthesized in the RER → post-translational modification in the Golgi → Packaged into secretory vesicles → exocytosis

Ligand binding Receptors

Ligands bind extracellular domain & cause conformational changes resulting in a physiological response: 1. Ligand-gated ion channels; --e.g., nicotinic, GABA, serotonin (5HT3), glycine 2. GPCR --e.g., adrenergic, muscarinic, angiotensin, vasopressin, oxytocin...... 3. Enzyme-linked receptors -- e.g. EGFR, NPR-A, receptor tyrosine kinase 4. Some 2nd messengers are ligands -- e.g., cAMP, cGMP, IP3, Ca2+, beta gamma-subunits are ligands that bind to intracellular receptors

What is the most important factor in determining if molecule can pass through lipid bilayer barrier?

Lipid to aqueous partition coefficient

How are peripheral membrane proteins attached attached to membrane?

Membrane loosely attached to integral membrane proteins via electrostatic interactions

How do membrane proteins insert into bilayers?

Membrane proteins go through a SORTING process & gather in the transGolgi network (TGN) so carrier vesicles I can insert them into their bilayers

Proteins within cytoskeletal network in cytosol from largest to smallest diameter:

Microtubules (tubulin)> intermediate filaments > microfilament (actin)

Phospholipids as substrates

Outer leaflet of cell membrane: - phosphatidylcholine - sphingomyelin Inner leaflet of cell membrane - Phosphatidylethanolamine - phosphatidylserine - phosphatdiylinisitol (PIP) *** PLC beta converts PHOSPHATIDYLINOSITOL 4,5 bisphosphate (PIP2)→ IP3 & DAG (phosphatidylserine + DAG + Ca2+ activates PKC) *** PI3kinase converts PIP2 to PIP3 *** PLA2 cleaves ARACHIDONIC ACID in the plasma membrane → → Prostaglandins & leukotrienes

What is the effector for Gq/GPC?

Phospholipase C(beta) (PLC [beta] ) is

What are products destined to leave the cell coated with in golgi?

Products destined to leave the cell are in secretory vesicles Given the proper stimulus this vesicle will fuse with the plasma membrane & extrude it's contents outside the cell by exocytosis.

Where is the 1st site along the SECRETORY pathway by secretory & membrane proteins before undergoing post-translational modification by the Golgi apparatus?

RER

2nd major class of membrane lipids

Sphingolipids -- derived from sphingosine instead of glycerol -- interact with phospholipid bilayer to prevent lipid molecules from diffusing away from membrane; e.g., sphingomyelin (D), galactocerebrosides (E) & gangliosides

Intermediate Filaments:

Unlike microtubules, they are stable and do not worry about hydrolysis

Most abundant protein in all cells

actin

Where are cytosolic proteins synthesized?

by free ribosomes in the cytosol

More abundant RER is found in what type of cell?

cells w/ greater secretory function ( i.e. pancreas)

Integral membrane proteins are embedded in the PM attached by ______

covalent bonds