C. Gel-like network
交错排列: In cell cortex
1.6 Functions of MFs
(1) Maintain cell’s shape and enforce PM
(2) Cell migration (Fibroblast)
Cell locomotion involves: Actin polymerization and Myosin I/II
Characteristics:
(1) Within a MF, all the actin monomers are oriented in the same direction, so MF has a polarity
(2) Dynamic equilibrium between the G-actin and polymeric forms, which is regulated by ATP hydrolysis and G-actin concentration.
Microtubules
Microfilamemts
Intermediate filaments
Figure 10-2. The three types of protein filaments that form the cytoskeleton.
Techniques for studying the cytoskeleton
MFs are organized into three forms by their binding proteins
A. 同向平行排列束
In: Microvillus and pseudopod
B: 收缩束
反向平行排列: in stress fiber, contractile ring, etc
封端（加帽）蛋白 单体-隔离蛋白
交联蛋白
成核蛋白
单体-聚合蛋白 成束蛋白
膜结合蛋白
纤维-切割蛋白 肌动蛋白纤维解聚蛋白
Nucleating proteins (成核蛋白)
Arp2/3 complex: Actin-related proteins
The modular structures of four actin-cross-linking proteins
(4) Dynamic equilibrium is required for the cell functions. Some MFs are temporary and others permanent.
microvilli
Contractile bundle in the cytoplasm
血影蛋白
丝束（毛缘）蛋白 辅肌动蛋白 细丝蛋白
The formation of two types of actin filament bundles: Contractile bundle mediated by α-actinin parallel bundle mediated by fimbrin.
A propsed mechanism for the movement of a nonmuscle cell in a directed manner.
Right: fluorescence micrograph of a portion of a cell infected with the bacterium L. monocytogenes. The bacteria appear as red-stained objects just in front of the greenstained filamentous actin tail. Left: electron micrograph of a cell infected with the same bacterium, showing the actin filaments that form behind the bacterial cell and push it through the cytoplasm.
ATPase
Mainly in muscle cells
Thick filamemts
Binding sites
Structure of a myosin II: contain one pair of heavy chains and two pairs of light chains
Motors Proteins that traverse cytoskeleton Motor protein（马达蛋白） convert chemical energy into mechanical energy, which is used to move cellular cargo attached to the motor. move unidirectionally along a cytoskeletal track in a stepwise manner, from one binding site to the next. three broad families: myosins（肌球蛋白）, kinesin（驱动蛋白）, and dynein（动力蛋白）
Drugs and mutations (about functions) Biochemical analysis (in vitro)
The self-assembly and dynamic structure of cytoskeletal filaments
Each type of cytoskeletal filament is constructed from smaller protein subunits. The cytoskeleton is a network of three filamentous structures. The cytoskeleton is a dynamic structure with many roles.
Chapter 9
Cytoskeleton System
Cytoskeleton (Narrow sense) A complex network of interconnected filaments and tubules that extends throughout the cytosol.
Ca2+ rise Ca2+ -calmodulin Bind to MLCK
(3) Microvillus: Support the projecting membrane of intestinal epithelial cells
Linkages Core actin filaments (by fimbrin and villin)
Actin filaments (rootlets) Spectrin connecting fibers
IF (keratin)
(4) Stress fibers
Composed of actin filaments and myosin-II
Focal contacts Stress Fibers
Response to tension
Focal contacts IFs
(5) Contractile ring: For cytokinesis
Sea urchin mesenchyme cell exposed to cytochalasin D
1.4 Actin-binding proteins （微丝结合蛋白）
The structures and functions of cytoskeleton are mainly controlled by its binding proteins
Sheet-like and fingerlike protrusions from the leading edge of a moving cell
Contractile ring during cell division
1.3 Specific drugs affect polymer dynamics
Fluorescent microscopy and Electron microscopy :
Immunofluorescence: fluorescently-labeled antibody Fluorescence: microinject into living cells Video microscopy: in vitro motility assays Electron: Triton X-100, Metal replica
III
I
N-terminus C-terminus
G-actin molecules polymerize to MF, also called F-actin
Exist in almost every type of eukaryotic cell, highly conserved
7 nmΒιβλιοθήκη 1.2 MF assembly and disassembly
(3) In vitro, (Polymerization) both ends of the MF grow, but the plus end faster than the minus. Because actin monomers tend to add to a filament’s plus end and leave from its minus end---- “Tread-milling”
1. Microfilaments (MFs)
1.1 Actin is the protein building block of MF