Cell Biology (BIO 320)
Actin filaments - analyze the data
(From: Molecular Cell Biology - Lodish et al)
Understanding of actin filaments has been greatly facilitated by the ability to purify actin
and actin-binding proteins and the ability to assemble actin filaments in vitro. Following
are some of the experimental approaches designed to characterize actin assembly and the
effects of actin-binding proteins on actin assembly.
1.
The following graph depicts the actin polymerization rate at the plus (+) and
minus (-) ends of rabbit actin as a function of actin concentration. Assume that
you could add actin filaments of a defined length to rabbit actin maintained at the
concentrations labeled A, B, and C in the figure. Describe the state of the
filaments (growth or shrinkage at each of the two ends) after a 10-minute
incubation at each of the indicated actin concentrations.
2.
An in vitro system was developed to study actin assembly and disassembly in
nonmuscle cells. In this study, tissue culture cells were incubated for several
hours with [35S]methionine so that all the actin monomers in each filament were
labeled. Actin filaments were then collected by differential centrifugation and
placed in a buffer containing one of three cytosolic extracts (A, B, or C). The
amounts of soluble actin in each sample were monitored over time as shown in
the following graph. What do these results tell you about the effects of A, B, and
C on the assembly and disassembly of actin filaments?
3.
A novel actin-binding protein (X) is overexpressed in certain highly malignant
cancers. You wish to determine if protein X caps actin filaments at the (+) or (-)
end. You incubate an excess of protein X with various concentrations of G-actin
under conditions that induce polymerization. Control samples are incubated in
the absence of protein X. The results are shown in the following graphs. How
can you conclude from these results that protein X binds to the (+) end of actin
filaments? Design an experiment, using myosin S1 fragments and electron
microscopy, to corroborate the conclusion that protein X binds to the (+) end.
What results would you expect if this conclusion is correct?