ACT-5 Is an Essential Caenorhabditis elegansActin Required for Intestinal Microvilli Formation

Investigation of Caenorhabditis elegans act-5gene function revealed that intestinal microvillus formation requires a specific actin isoform. ACT-5 is the most diverged of the five C elegansactins, sharing only 93% identity with the other four. Green fluorescent protein reporter and immunofluorescence analysis indicated that act-5gene expression is limited to microvillus-containing cells within the intestine and excretory systems and that ACT-5 is apically localized within intestinal cells. Animals heterozygous for a dominant act-5mutation looked clear and thin and grew slowly. Animals homozygous for either the dominant act-5mutation, or a recessive loss of function mutant, exhibited normal morphology and intestinal cell polarity, but died during the first larval stage. Ultrastructural analysis revealed a complete loss of intestinal microvilli in homozygous act-5mutants. Forced expression of ACT-1 under the control of the act-5promoter did not rescue the lethality of the act-5mutant. Together with immuno-electron microscopy experiments that indicated ACT-5 is enriched within microvilli themselves, these results suggest a microvillus-specific function for act-5 and further, they raise the possibility that specific actins may be specialized for building microvilli and related structures.