Functions of LIM proteins in cell polarity and chemotactic motility.

LimC and LimD are two novel LIM proteins of Dictyostelium, which are comprised of double and single LIM domains, respectively. Green fluorescent protein-fused LimC and LimD proteins preferentially accumulate at areas of the cell cortex where they co-localize with actin and associate transiently with cytoskeleton-dependent dynamic structures like phagosomes, macropinosomes and pseudopods. Furthermore, both LimC and LimD interact directly with F-actin in vitro. Mutant cells that lack either LimC or LimD, or both, exhibit normal growth. They are, however, significantly impaired in growth under stress conditions and are highly sensitive to osmotic shock, suggesting that LimC and LimD contribute towards the maintenance of cortical strength. Moreover, we noted an altered morphology and F-actin distribution in LimD(-) and LimC(-)/D(-) mutants, and changes in chemotactic motility associated with an increased pseudopod formation. Our results reveal both unique and overlapping roles for LimC and LimD, and suggest that both act directly on the actin cytoskeleton and provide rigidity to the cortex.