Introduction to Biological Sciences 110a In class notes for October 10, 2007 The cytoskeleton Maintains cell structure (acts as a molecular skeleton) Microfilaments are concentrated around the nucleus, they are also often found beneath the plasma membrane Used for movement Used to apply force Used for intracellular movement Structure and motility Three kinds of filaments (made of protein assemblies) Microtubles Microfilaments <a href="/keyword/intermediate-filaments/" >intermediate filaments</a> Microtubles The largest in diameter of the cytoskeletal filaments They have a diameter of about 25 nm, or 250 They are hollow structures The space in the middle is about 170 across They are made up of rows of protein subunits Each row is called a protofilament Most have 13 protofilaments Each protofilament is made of rows of protein subunits There are two types of protein subunits: Alpha tublin Beta tublin These are very similar Alpha and Beta tublin both bind a molecule of GTP Alpha binds it irreversibly (they use it as a prosthetic group) Beta binds it exchangebly (it can come off or on repeatedly) and can hydrolzye and for GDP GTP regulates microtuble assembly (decides if it will be in a microtuble or floating around in the cytosol) GDP makes it shorter GTP lets it grow Look up signal recognition particle on ER. Look up vesicle coat protein assembly. GTP is involved in both of these Microbules are structually polar They have physically different ends They are considered to have + and ends On the slide ??? about what end is what A singlet microtuble is the one we have been describing In some more complicated microtubles there are doublet and triplet forms. Double They have 13 microfilaments for one ring then 10 more form the second ring (A and B tubles) Triple They have 13 for one ring, then two rings of 10 (called A , B, and C tubles) Introduction to Biological Sciences 110a In class notes for October 10, 2007 Microtubles assemble and reassemble according to the life of the cell They tend to grow from organizing center in the middle of the cell (MTOCs) Most commonly they are in the center and are called centrosomes A centrosome consists of 2 centrioles and a pericentriolar matrix (stuff around the centrioles) Centrioles are made of microtubles and consist of 9 triple microtubles Microtubules radiate from centrosomes Microtubles in the cells grow from nucleating centers in the pericentriolar matrix. The other type of tublin (gamma) forms rings of tublin nucleate assembly. See A Microtubles have a positive and negative end The negative end is toward the centrosome The cell regulate MT growth by altering the stability (stable, grows, unstable, disassembles The cell produces other proteins that bind to the microtubles Some of the proteins are called microtuble associated proteins (MAPS) MAPS stabilize the microtuble structures They also can connect MTs with other MTs and other filaments Some can link to intermediates, then the intermediates link to microtubles These are not considered to be maps Some proteins are involved with intracellular motility Act as tracks in cells (like a train track) Vesicles can go on this Motor proteins bind the vesicles to the tracks They have binding domains At the end of the microtuble protein there are heads that move along the MT (they walk along the MT) It takes energy to move things around in the cell. The energy comes from ATP. Two types of motorproteins have been discovered for microtubles Kinesin Usually involved in moving things to the positive end of microtuble Dynein Usually involved in moving things to the negative end of microtuble Specialized micrtotuble structure Introduction to Biological Sciences 110a In class notes for October 10, 2007 Cilia (or cillium, singular) are filamentous extension contains 9 doublet microtubles grouped around two singlet microtubles the doublet microtubles are linked to their neighbors by dynein proteins the doublet microtubles are anchored in the basal body (like a centriole, a ring of 9 triplet microtubles, organizing center) the microtubles bend(dynein pulls only on one side), producing a waving motion, this propels liquid across the cells smoking destroys the cilia in the lungs Microfilaments Much smaller diameter, only about 80 or 8 nm They are not hollow Consist of two strands that are made from subunits of a protein called actin Actin, when free in solution, is called G-actin, in a filiament F-actin Actin and tublin are free in the cell in their subunits in dynamic equillibrium with the filaments Assembly Requires ATP, instead of GTP Have a plus end and minus end Actin-ATP subunits attach ATP is hydrolyzed to ADP Actin filaments have specialized functions that depend on what is bound to them Actin-binding proteins Functions Nucleate filament formation Can start or block the ends Cross-link different microfilaments, making networks (like a plaid structure) Others organize them into parallel strands Others sever the filaments Some bind to membranes Mysoine is the most studied The motor protein for Actin ATP dependent Moves things around There are many types of myosine They fall into the catagories of muscle myosine and nonmuscle mysoine. Introduction to Biological Sciences 110a In class notes for October 10, 2007 Develop force and movement or keeping the filaments under tension <a href="/keyword/intermediate-filaments/" >intermediate filaments</a> Between the sizes of the other two only found in animal cells so far, the other two were found in all eukaryotic diameter is not fixed but is around 10 nm The protiens that the intermediate proteins are made up of are different in different tissues (they are tissue specific proteins) They vary greatly in size They all have an alpha helical core The rest of the protein varies More than 50 different proteins have been found to form these filiaments They alpha helical core consists of coil coil alpha helices <a href="/keyword/intermediate-filaments/" >intermediate filaments</a> are not polar There are 32 chains of alpha helices stacked together <a href="/keyword/intermediate-filaments/" >intermediate filaments</a> are not involved in any motility They are involved in functions that take advantage of the fact that they are very very stable They are very strong Their functions tend to be structural, supportive, and protective They interact with other filaments and structures through associated proteins Example Plectin in cross-bridges Keratin is the intermediate filament protein that we find in cells that give rise to skin, hair, and nails Filaments form protective layers of keratin, they are strong because the protofilaments are cross-linked by disulfide bridges. Reducing agents can break these disulfide bridges in the <a href="/keyword/intermediate-filaments/" >intermediate filaments</a> .