The C. elegans M lineage is useful as a model for studying mesodermal patterning and cell fate specification. The M lineage arises from a single precursor cell, the M mesoblast, during embryogenesis. In hermaphrodites, M undergoes a series of postembryonic divisions to form 14 body wall muscles (BWMs), 2 coelomocytes (CCs), and 2 sex myoblasts (SMs). In a screen for mutants affecting the proper development of the M lineage (mesodermal lineage specification or mls mutants), we isolated four mutations of the gene
fozi-1. We used a combination of cell-type specific markers to follow the M lineage in
fozi-1 mutants and found that
fozi-1 is necessary for proper specification of the CC and BWM fates. We mapped and cloned
fozi-1 and found that it encodes a novel protein with a glutamine rich region and two C2H2 zinc finger motifs (two motifs characteristic of transcription factors), as well as a large C-terminal FH2 Formin Homology 2 domain (characteristic of a large family of actin binding proteins). Thus we have named this gene
fozi-1(formin zinc finger protein-1). The FH2 domains in other formins have been shown to function by binding to actin. We found that the FH2 domain in FOZI-1 is quite divergent from all other formins and has no actin binding activity in vitro.
We have generated antibodies specific to FOZI-1 and found that FOZI-1 is a nuclear protein expressed within the M lineage at the time of CC and BWM fate specification. Using the
hlh-8 promoter to express
fozi-1 exclusively in all undifferentiated cells of the M lineage, we were able to rescue the M lineage phenotypes of
fozi-1 mutants. Therefore,
fozi-1 functions within the M lineage to specify proper CC and BWM fates. Since the M lineage expression pattern and mutant phenotypes of
hlh-1(encoding CeMyoD) are very similar to what we have observed for
fozi-1, we investigated the relationship between these two genes. We found that
fozi-1 does not affect the expression or subcellular localization of HLH-1, nor does
hlh-1 affect FOZI-1 expression and localization.
hlh-1 and
fozi-1 mutants also failed to show any genetic interaction, suggesting that FOZI-1 and HLH-1 are likely to function independently to regulate CC and BWM fate specification. We are currently testing if
fozi-1 is sufficient to produce CC and BWM fates when ectopically expressed.