Active transport of dampers. (a) Damper proteins
(red) can be actively transported on a filament by motors
(orange) and delivered to an elongator
(green), where they inhibit the activity of the elongator.
An example is the case of Smy1 proteins in budding yeast, which are delivered to
the formin at the barbed end of the actin cable by myosin motors. Smy1 bound to
formin inhibits its polymerization activity. (b) Longer cables
will encounter more dampers and hence will provide more inhibition to the
elongator as compared to shorter ones, thus setting up a negative feedback. This
process leads to a length-dependent assembly rate (blue). In
concert with a constant disassembly rate (green), these
filament dynamics lead to a peaked length distribution in steady state
(c). The parameter values used are r
= 370/s, γ = 45/s, w
= 0.004/s (kon =
wl), and koff =
1/s, and they correspond to actin cables in budding yeast
(8, 40).