Lectures 16 and 17

Muscle and motility


Muscle Structure:


A muscle consists of thousands of muscle fibers, the cellular units of muscle.

Muscle Fiber

Each muscle fiber is made up of thousands of myofibrils.


Myofibrils contain filaments of actin and myosin.  The filaments form an ordered array and make up sarcomeres, the functional units of muscle.

Actin and Myosin

According to the sliding filament theory, myosin heads bind actin filaments and move them during contraction.  The micrograph shows myosin bound to actin, supporting this theory.

(Table borrowed from UCSF Muscle Page)

"Seen in longitudinal section (with an electron microscope), a myofilament shows several distict bands, each of which has been given a special letter. The lightest (least electron dense) band is known as the I band and consists mostly of actin. The wide, dark band, known as the A band, is composed primarily of myosin. In the center of the I band is an electron dense line, known as the Z-line. In the middle of the A band is another dense line known as the M line." (From Myofilament Structure link above).

Myosin binding to actin (from Rayment et al.)

The cross-bridge cycle


Structure of myosin, and PDB files

2mys.pdb, Structure of myosin head, and minor subunits (no nucleotides).
1mmg.pdb, Structure of myosin head, with ATP analog (ATP-gamma-S) in situ.
1mma.pdb, Structure of myosin head, with ADP in situ.
(These files show conformational changes on binding adenine nucleotides.)

The University of Cambrige Myosin Home Page, - many nice images, and much useful information, including:


"At least six different types of actin are synthesized by vertebrate cells. The different genes which code them are highly conserved however, and they have similar properties. Globular, or G actin is stabilized by one tightly bound calcium ion. The protein is also associated with one molecule of ATP. The G actin polymerizes to form filamentous or F actin; this involves hydrolysis of the terminal phosphate of the ATP. Actin filaments consist of two strands of polymerized actin monomers, twisted into a helix with 13.5 molecules per turn." (From introduction to actin filaments reference below.)

Structure of actin, and PDB files

Actin-profilin complex (1hlu.pdb file)
Actin-DNA-ase complex (1atn.pdb file)

Muscle Links

Links to other muscle sites can be found on Washington State University Muscle page.


Introduction to Cell Biology, including a nice page on microtubules, with extensive graphics.
A brief description of the role of the cytoskeleton in mitosis

Click here for a movie on the dynamic redistribution of the cytoskeleton during phagocytosis from Cells Alive.

Description. "The cytoskeleton determines the shape of a cell, much like our bones help shape our bodies. In addition, the cytoskeleton makes the cell move, more like our muscles do. Like in muscle, this role is fulfilled in single cells in particular by the proteins actin and myosin. With the help of molecular genetics, a protein of the cytoskeleton called "coronin" that co-distributes with actin was labelled with green fluorescent protein, which is observable in cells like the living Dictyostelium discoideum shown above.
We have recorded the redistribution of coronin with a confocal microscope, which produces an image of a thin slice of the cell, like looking at only the beef within a hamburger. At regions which protrude from the surface of the cell, the so-called pseudopods, the labelled cytoskeletal protein accumulates transiently. This is also the case at a site where a yeast cell, marked with a red dye, is taken up by phagocytosis. After the particle is swallowed, the cytoskeletal protein returns to its original position beneath the cell surface." (From the Cells Alive page.)

An archive showing various aspects of cell biology in movies, including chloroplast movement along the actin (or actin-myosin) cytoskeleton (look under Plastids, or under Motility).

Cilia and eukaryote flagella

( Animation and Figs. from Molecular Machines)

A nice page on cilia and eukaryote flagella.
The AXONEMAL DYNEIN family of proteins -- Summary


"3-D view of a neuron with its processes containing microtubules. At higher magnifications, the vesicles are seen attached to microtubule associated proteins (MAPs) and moving along the microtubule conveyer belt. The MAPs include kinesin and dynein which "walk" along the microtubules in opposite directions. Kinesin moves the vesicle along towards the plus end and dynein walks towards the minus end. In neurons, as the microtubules grow from the cell body through the processes, the plus end is more peripheral." (From the microtubules page above.)

Visit the Kinesin Home Page, including: