09. MODULE 9.pdf

MODULE 9 Proteins Binding Actin Subunits Actin monomer concentration in cells exceeds the critical level needed for polymerization, but polymerization does not occur due to monomer sequestration. Actin monomers bound to thymosin do not polymerize. Profilin binds to an actin monomer and blocks the...

MODULE 9 Proteins Binding Actin Subunits Actin monomer concentration in cells exceeds the critical level needed for polymerization, but polymerization does not occur due to monomer sequestration. Actin monomers bound to thymosin do not polymerize. Profilin binds to an actin monomer and blocks the minus end, competing with thymosin. Profilin moves actin subunits from the thymosin-bound pool to the plus end of filaments. Advantage: Profilin helps maintain a large pool of subunits, enabling fast filament growth. Gelsolin and Cofilin Cofilin disassembles older ADP-bound actin filaments free of tropomyosin. It binds preferentially to actin with ADP and introduces strain by twisting the filament. Gelsolin, regulated by calcium, binds to the plus ends of actin monomers and filaments, preventing monomer exchange. Gelsolin plays a role in both the assembly and disassembly of actin filaments. Proteins Binding Tubulin Subunits Stathmin binds tubulin dimers and decreases the elongation rate of microtubules by preventing addition at their ends. Slowing elongation increases microtubule shrinkage and dynamic turnover. Microtubule-Associated Proteins (MAPs) Stabilize the minus end of microtubules, aiding in the formation of new microtubules. Involved in transport and sliding of microtubules with the help of motor proteins like kinesins and dyneins. Concentrate microtubules in specific areas during mitosis (e.g., spindle poles). Some MAP complexes stabilize, destabilize, or promote microtubule polymerization. Microtubule-severing proteins can cause the formation of two new microtubules by unfolding tubulin monomers in the middle of a microtubule. Proteins at the Filament Ends Actin filaments are stabilized by capping proteins like CapZ at the plus end and the ARP complex at the minus end. In muscle cells, actin filaments are capped by CapZ at the plus end and tropomodulin at the minus end. The minus end of microtubules is capped by the γ-tubulin ring complex (γ-TuRC), which also nucleates microtubules. MODULE 9 Catastrophin proteins bind to microtubule ends and destabilize them, increasing microtubule shrinkage. Intermediate Filaments Intermediate filaments (IFs) are present in some eukaryotic cells but not all, larger than microfilaments but smaller than microtubules (10nm in diameter.) Provide structural stability to the cytoplasm and are hard to break due to their rope-like structure. Nuclear lamins provide strength and support to the nucleus. Intermediate Filaments Structure Two monomers form a coiled-coil dimer. Two dimers form an antiparallel tetramer, allowing association with other tetramers. Tetramers have no polarity, unlike microtubules and actin filaments. Intermediate Filaments (IFs) Somewhat dynamic, phosphorylation may regulate their disassembly. Keratins: a single epithelial cell may produce multiple types of keratins, but these copolymerize into a single network. A keratin filament is made of type I (acidic) and type II (neutral/basic) keratin chains to form dimers, two of which join to form a tetrameric subunit. Neurofilaments are found along axons of neurons. Vimentin-like filaments in muscle cells, etc. Putting It Together Proteins that bind free building blocks of filaments influence the filaments' polymerization rate. Microtubule-associated proteins regulate microtubule (MT) dynamics. Proteins binding filament ends affect their stability. Intermediate filaments have tissue-specific forms, including keratins in epithelial cells, neurofilaments in neurons, and desmins in muscle cells. Intermediate filaments are rope-like, easy to bend, hard to break.