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Ponticulin

Updated 28/7/03

A membranous actin-binding protein originally isolated from Dictyostelium but now known to exist in vertebrates (Wuestehube & Luna, 1987; Wuestehube et al, 1989; Luna et al, 1990). Ponticulin is a trans-membranous protein (Chia et al, 1993, Hitt et al, 1994a, 1994b), that constitutes a major link between the cell membrane and the cortical actin cytoskeleton (Hitt et al, 1994b). It is relatively abundant at about 1 million copies per Dictyostelium amoeba corresponding to a density of 300 per mm2 (Chia et al, 1993).  The linking function is responsible for the name "Ponticulin", derived from the Latin for a small bridge (ponticulus).  

Structure
Ponticulin appears to exist as a monomer within the membrane (). The primary sequence does not seem to be related to other proteins and Blast searches do not reveal the existence of similar proteins.  This could be because of low abundance of mRNA, the small size and low conservation (see iso-forms below).  No regions of the protein appears to contain typical actin binding motifs.

Isoforms
Two iso-forms of ponticulin are expressed in Dictyostelium, in addition to the original described gene product (now called ponA), a second form ponB has been identified in cells grown on bacteria and axenic amoebae in the fusion-competent stage (
Hitt et al, 2003).  There is only 50% similarities and 37% identity between the two iso-forms but the six cysteines are conserved and the proteins share similar hydrophobicities (Hitt et al, 2003).

Nucleating Activity
The protein has been described as having actin polymerising nucleating activity (
Chia et al, 1993).  

A role in adhesion?
Ponticulin becomes localized to tritonX-100 insoluble rafts in association with the adhesion protein gp80 (Harris et al, 2003) at cell -cell contacts even if the cytoskeleton is disrupted by latrunculin.  In Dictyostelium both actin and myosin II were found to be associated with these rafts. 

References:-

Chia, C. P., Hitt, A. L. & Luna, E. J. (1991) Direct binding of F-actin to ponticulin, an integral plasma membrane glycoprotein. Cell Mot Cytoskeleton. 18, 164-179.

Chia, C. P., Shariff, A., Savage, S. A. & Luna, E. J. (1993) The integral membrane protein, ponticulin, acts as a monomer in nucleating actin assembly, J. Cell Biol. 120, 909-922.

Harris, T. J. C., Ravandi, A., Awrey, D. E. & Siu, C.-H. (2003) Cytoskeleton interactions involved in the assembly and function of glycoprotein-80 adhesion complexes in Dictyostelium. J. Biol. Chem. 278, 2614-2623.

Hitt, A. L., Lu, T. H. & Luna, E. J. (1994a) Ponticulin is an atypical membrane protein, J. Cell Biol. 126, 1421-1431.

Hitt, A. L., Hartwig, J. H. & Luna, E. J. (1994b) Ponticulin is the major high affinity link between the plasma membrane and the cortical actin network in Dictyostelium, J. Cell Biol. 126, 1433-1444.

Hitt, A. L., Iijima-Shimizu, M., DuBay, M. J., Antonette, L. L., Urushihara, H. & Wilkerson, C. G. (2003) Identification of a second member of the ponticulin gene family and its differential expression pattern. BBA. 1628, 79-87.

Luna, E. J., Wuestehube, L. J., Chia, C. P., Shariff, A., Hitt, A. L. & Ingalls, H. M. (1990) Ponticulin, a developmentally-regulated plasma membrane glycoprotein, mediates actin binding and nucleation. Dev.Genet. 11, 354-361.

Shariff, A. & Luna, E. J. (1990) Dictyostelium discoideum plasma membrane contain an actin-nucleating activity that requires ponticulin, an integral membrane glycoprotein. J.Cell Biol. 110, 681-692.

Shutt, D. C., Wessels, D., Wagenknecht, K., Chandrasekhar, A., Hitt, A. L., Luna, E. J. & Soll, D. R. (1995) Ponticulin plays a role in the positional stabilization of pseudopods, J.Cell Biol. 131, 1495-1506.

Wuestehube, L. J. & Luna, E. J. (1987) F-actin binds to the cytoplasmic surface of ponticulin, a 17-kD integral glycoprotein from Dictyostelium discoideum plasma membranes. J.Cell Biol. 105, 1741-1751.

Wuestehube, L. J., Chia, C. P. & Luna, E. J. (1989) Indirect immunofluorescence localization of ponticulin in motile cells. Cell Mot Cytoskeleton. 13, 245-263.

Wuestehube, L. J., Speicher, D. W., Shariff, A. & Luna, E. J. (1991) F-actin affinity chromatography of detergent-solubized plasma membranes: purification and initial characterization of ponticulin from Dictyostelium discoideum. Methods Enzymol. 196, 47-65.

 
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