BIO 211 Lecture Notes - Lecture 4: Profilin, Colchicine, Myosin Light-Chain Kinase
30 May 2018
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Prior to Week 4 Class 1 – pp. 889-904
Know
• The basic functions of the cytoskeleton
o its varied functions depend on the behavior of three families of protein filaments:
actin filaments, microtubules, and intermediate filaments
o hundreds of accessory proteins
o can be regulated by direct covalent modification of filament subunits, but usually
accessory proteins are used instead
▪ determine distribution and behavior of filaments based on signals
▪ bring the cytoskeleton under control of intra and extracellular signals
(including signal to divide cell)
▪ cool!: motor proteins
• bind to a polarized filament and use ATP hydrolysis to move along
it
• depends on whether actin or tubulin, and depends on their cargo
• ex: moving entire organelles to where they need to go
• What the three major types of protein filaments that form the cytoskeleton are, and their
basic properties
o actin filaments
▪ determine the shape of the cell’s surface
▪ drive the pinching of one cell into two
o microtubules
▪ determine the positions of membrane-enclosed organelles
▪ direct intracellular transport
▪ form the mitotic spindle that segregates chromosomes during cell division
o intermediate filaments
▪ provide mechanical strength
• The protein subunits that make up the major cytoskeletal filaments
o form as helical assemblies of subunits that self-associate, using a combo of end-
to-end and side-to-side protein contacts
o actin filaments
▪ actin subunits
▪ compact and globular
▪ asymmetrical and bind to one another head-to-tail so that they all point in
the same direction → polarity
▪ are enzymes that get energy from ATP synthesis
▪ formed in such a way that their ability to pull and move is utilized
o microtubules
▪ tubulin subunits
▪ everything else is the same as actin
▪ but also protofilaments (see below)
o intermediate filaments
▪ smaller subunits that are themselves elongated and fibrous
▪ symmetrical and not polarized and not enzymes
▪ but can be assembled really quickly by kinases when required
find more resources at oneclass.com
find more resources at oneclass.com
o their covalent bonds are strong, they have weaker intra-fibrous bonds, and they
can make and break chains without forming covalent bonds
• That bacterial cells also depend on cytoskeletal filaments
o bacterial cell organization and division depend on homologs of eukaryotic
cytoskeletal proteins
o FtsZ: homolog of tubulin, helps w cell division
o MreB and Mbl: homolog of actin, help w cell shape
o ParM: specialized homolog of actin and MreB/Mbl
▪ similar: TubZ
o Caulobacter has one similar to intermediate → helps w shape
• What nucleation is
o nucleation is the rate-limiting step in the formation of actin filaments
o for new filaments, subunits must form a aggregation, or nucleus, before they can
elongate rapidly
o barrier: the instability of a small actin filament → but a few of the aggregates get
it going and then it’s a really fast chain reaction
o cells use special proteins to catalyze filament nucleation at specific sites to
determine where actin filaments are formed
o see critical concentration for related
Understand
• Why it is important that the cytoskeleton is dynamic and adaptable
o large-scale cytoskeletal structures can change or persist, according to need, lasting
for any length of time
o the individual components that make up the structures are in a constant state of
flux
o structural rearrangement in a cell requires little energy when conditions change
o regulation allows cells to build an enormous range of structures from three basic
types
▪ then there are a bunch of different possible uses for this stuff so maybe
read that, lots of examples
▪ actin for movement but needs to reorganize to move in a different way
o cell division!
▪ microtubules form the mitotic spindle
▪ actin forms a contractile ring
• When it might be important to have stable cytoskeletal structures
o the cytoskeleton determines cellular organization and polarity
o for differentiated cell there must be long term structures so that the cell can
organize around them
o examples
▪ the actin bundles of hair cells must remain stable over their entire lifetime,
but this does not mean individual actin molecules are not dynamic
o responsible for polarity: organized arrays of microtubules, actin, and
intermediates maintain differences between the apical and basolateral surfaces
• Why protofilaments are important
find more resources at oneclass.com
find more resources at oneclass.com
o a thousand tubulin subunits can span a cell but they don’t have the strength! to
provide both speed and strength, microtubules are made of 13 protofilaments
o loss of a subunit at the end breaks only a few bonds, but breaking off a subunit at
the middle breaks a lot
o allows microtubules to be strong while also allowing rapid changes to the ends
• The roles of the bacterial cytoskeletal elements FtsZ and MreB/Mbl and ParM
o FtsZ
▪ can polymerize into filaments and assemble into a ring (Z-ring) at the site
where the septum forms during cell division
▪ everything is highly dynamic like actin ring
▪ the filaments are thought to generate the necessary bending force
▪ may also call enzymes over
o MreB and Mbl
▪ found in rod/spiral shaped cells
▪ assemble to form dynamic patches that move circumferentially along the
cell
▪ serve as a scaffold for the peptidoglycan cell wall to protect cell shape
▪ chromosome segregation?
o ParM
▪ found on a plasmid
▪ assembled into filaments that associate at each end with a copy of the
plasmid
▪ pushed plasmid copies apart
▪ very specialized? selective stabilization
• How actin subunits assemble
o actin subunits assemble head-to-tail to create flexible, polar filaments
▪ form a tight, right-handed helix
▪ 8 nm wide → F-actin
o slower growing minus end and faster growing plus end due to the polarity (see
more about this below)
o thinner and need less energy to break, but are often bundled inside cells
o individual actin filaments are quite flexible
▪ the stiffness of a filament can be characterized by its persistence length
(minimum length at which thermal fluctuation cause bending)
▪ quite small persistence length, but accessory proteins cross-link and
bundle the filaments together
• What critical concentration is, and why adding preexisting seeds to a solution of
cytoskeletal monomers can promote polymer formation
o as the concentration of free subunits declines, the system approaches a rate where
subunits assembled = subunits disassembled
o a bunch of number stuff
o the lag phase in filament growth is eliminated if pre-existing seeds are added to
the solution at the beginning of polymerization
▪ seeds: fragments of actin filaments that have been chemically cross-linked.
o the kinetic rate constants are much greater at the plus end than at the minus end
• The difference between the plus and minus ends of actin filaments
find more resources at oneclass.com
find more resources at oneclass.com
