Membrane nanotubes are cell protrusions that grow to tens of micrometres and functionally
connect cells. Actin filaments are semi-flexible polymers, and their polymerisation
provides force for the formation and growth of membrane nanotubes. The molecular bases
for the provision of appropriate force through such long distances are not yet clear.
Actin filament bundles are likely involved in these processes; however, even actin
bundles weaken when growing over long distances, and there must be a mechanism for
their regeneration along the nanotubes. We investigated the possibility of the formation
of periodic molecular relay stations along membrane nanotubes by describing the interactions
of actin with full-length IRSp53 protein and its N-terminal I-BAR domain. We concluded
that I-BAR is involved in the early phase of the formation of cell projections, while
IRSp53 is also important for the elongation of protrusions. Considering that IRSp53
binds to the membrane along the nanotubes and nucleates actin polymerisation, we propose
that, in membrane nanotubes, IRSp53 establishes molecular relay stations for actin
polymerisation and, as a result, supports the generation of force required for the
growth of nanotubes.
