Carnell et al. have discovered a new job for actin. In social amoebas, actin helps recycle a valuable protein before the cell throws out its garbage.

Dictyostelium is not a picky eater. So sometimes the social amoeba gobbles something it can't digest, such as yeast or the cell walls of certain bacteria, and ends up spitting out its meal. In this process, the lysosome that harbors the indigestible material switches from acidic to neutral and converts into a postlysosome that eventually jettisons its contents through exocytosis. A layer of polymerized F-actin coats the lysosome as it neutralizes, but an open question was actin's function during the transformation.

Carnell et al. discovered the answer while studying the role of a lysosomal protein called WASH that regulates actin polymerization. Exocytosis stalled in slime mold cells lacking WASH. If the cells consumed large amounts of indigestible dextran, they ballooned and sported huge lysosomes that couldn't be expelled. Without WASH, lysosomes didn't neutralize, and cells didn't accumulate the large, actin-covered vesicles that form when several postlysosomes fuse before exocytosis.

Removing WASH also prevented cells from recycling V-ATPase, a protein complex that attaches to the lysosome and maintains the high acidity inside. The researchers think that WASH's role on the lysosome is to attract actin that ushers V-ATPase (and presumably other molecules) into recycling endosomes. V-ATPase is large and costly to make, so the cell gains by recycling it from the lysosome rather than misdirecting it to the plasma membrane.


et al
J. Cell Biol.