PONTICULIN PLAYS A ROLE IN THE POSITIONAL STABILIZATION OF PSEUDOPODS

Ponticulin is a 17-kD glycoprotein that represents a major high affini ty link between the plasma membrane and the cortical actin network of Dictyostellium. To assess the role of ponticulin in pseudopod extensio n and retraction, the motile behavior of two independently generated m utants lacking ponticulin was analyzed using computer-assisted two- an d three-dimensional motion analysis systems. More than half of the lat eral pseudopods formed off the substratum by ponticulin-minus cells sl ipped relative to the substratum during extension and retraction. In c ontrast, all pseudopods formed off the substratum by wild-type cells w ere positionally fixed in relation to the substratum, Ponticulin-minus cells also formed a greater proportion of both anterior and lateral p seudopods off the substratum and absorbed a greater proportion of late ral pseudopods into the uropod than wild-type cells. In a spatial grad ient of cAMP, ponticulin-minus cells were less efficient in tracking t he source of chemoattractant. Since ponticulin-minus cells extend and retract pseudopods with the same time course as wild-type cells, these behavioral defects in ponticulin-minus cells appear to be the consequ ence of pseudopod slippage, These results demonstrate that pseudopods formed off the substratum by wild-type cells are positionally fixed in relation to the substratum, that ponticulin is required for positiona l stabilization, and that the loss of ponticulin and the concomitant l oss of positional stability of pseudopods correlate with a decrease in the efficiency of chemotaxis.