Cofilin

ADF/cofilin is a family of actin-binding proteins that disassembles actin filaments. Actin-binding proteins regulate assembly and disassembly of actin filaments (Cooper et. al., 2004). Cofilin, a member of the ADF/cofilin is actually a protein that has 70% sequence homology to ADF, making it part of the ADF/cofilin family of small ADP-binding proteins (McGough et. al., 1997). The protein binds to actin monomers and filaments, G actin and F actin, respectively (Lappalainen et. al., 1997). Cofilin causes depolymerization at the minus end of filaments, thereby preventing their reassembly. The protein is known to sever actin filaments by creating more positive ends on filament fragments (Cooper et. al., 2004). Cofilin/ADF(destrin) is likely to sever F-actin without capping (McGough et. al., 1997) and prefers ADP-actin. These monomers can be recycled by profilin, activating monomers to go back into filament form again by an ADP to ATP exchange. ATP-actin is then available for assembly (Cooper et. al., 2004).

Structure

Cofilin alters F-actin structure to make it twisted. The structure is a helix, proposed to bind G-actin. ADF/Destrin fits better with a twist in F-actin between two actin subunits (see figure above). The levels of cofilin are shown in 'd' above. 4 indicates 40% and 1 indicates 10% by volume of cofilin. The silver portion of image 'd' is actin. The cofilin binding site includes subdomain 2. The twist in the structure causes strain at the actin-actin contact site. Four actin histidines near the cofilin binding site may be needed for cofilin/actin interaction, but pH sensitivity alone may not be enough of an explanation for the levels of interaction encountered. Cofilin is accommodated in ADP-F actin because of increased flexibility in this form of actin. Binding by both cofilin and ADF(destrin) causes the crossover length of the filament to be reduced. Therefore, strains increase filament dynamics and the level of filament fragmentation observed (McGough et. al., 1997).

Function

Cofilin brings two filaments of actin together by joining actin subunits. The mechanism by which cofilin affects actin dynamics is controversial. There is apparent cooperative binding to F-actin, allowing for twisting to take place. The twisting is favorable for forming actin bundles because there is less fragmentation when in a twisted conformation (McGough et. al., 1997).

What Cofilin Functions with

The Arp2/3 complex and cofilin work together to reorganize the actin filaments in the cytoskeleton. Arp 2/3, an actin binding complex of proteins, binds to actin-ATP at the plus ends of filaments, causing a new branch of filamentous actin to begin (Cooper et. al., 2004), while cofilin depolymerization takes place after dissociating from the Arp2/3 complex. They also work together to reorganize microtubules in order to traffic more proteins by vesicle to continue the growth of filaments (Svitkina et. al., 1999).

Cofilin also binds with other proteins such as myosin, tropomyosin, α-actinin, gelsolin and scruin. Those proteins and compete with cofilin to bind to actin (McGough et. al., 1997).

Effects on Function

The ADF/cofilin family of actin-binding proteins binds with G-actin monomers and depolymerizes F-actin (Carlier et. al., 1997). They also bind to F-actin and may sever filaments, when the pH changes within the cell (Carlier et. al., 1997). Along with depolymerizing in a pH dependent manner, cofilin is also found to be temperature sensitive. More cofilin is needed at higher temperatures because more depolymerization occurs as the protein falls apart. Thereofore, pH, phosphorylation and phosphoinositides regulate cofilin’s binding and associating activity with actin (Lappalainen et. al., 1997).

In a Model Organism

ADF/cofilins are found in ruffling membranes and at the leading edge of mobile cells (Carlier et. al., 1997). In particular, ADF/cofilin promotes disassembly of the filament at the rear of the brush in Xenopus laevis lamellipodia, a protrusion from fibroblast cells characterized by actin networks. Subunits are added to barbed ends and lost from rear-facing pointed ends. Increasing the rate constant, k, for actin dissociation from the pointed ends was found to sever actin filaments. Through this experimentation, it was found that ATP or ADP-Pi are probably involved in binding to actin filaments (Svitkina et. al., 1999).

References

Carlier, M. F., Laurent, V., Santoloni, J., Melki, R., Didry, D., Xia, G. X., Hong, Y., Chua, N. H., and D. Pantaloni (1997) The Journal of Cell Biology 136:1307-1323.

Cooper, G. M. and R. E. Hausman. The Cell: A Molecular Approach, 3rd ed. Washington DC: ASM Press 2004 pp.436-440.

Lappalainen, P. and D. G. Drubin (1997) Nature 388:77-82.

McGough, A., Pope, B., Chiu, W., and A. Weeds (1997) The Journal of Cell Biology 138:771-781.