lon-2 mutants are 1.5 times longer than N2, but do not generate 1.5 times more cells than wild-type (Freyd and Horvitz, WBG 10#3p58). This suggests that
lon-2 animals are long either because of defects in embryonic elongation or abnormalities in cuticle structure. Savage, Baird, and Padgett (WBG 13#2p38) have suggested that
lon-2 may downregulate the TGF-b-like signalling pathway defined by
daf-4, and
sma-2,3,4 and 6, which regulates body size. While cloning
bor-1 X we performed Southern blot analysis of
lon-2(
n1630) X, and of two wild-type revertants of this Tc1 allele (isolated by G. Freyd and Bob Horvitz). Using the cosmid F35B9 as a probe, we detected a Tc1-sized insertion in
n1630 that was absent in the revertants. Sequence analysis showed that the Tc1 insertion was in the second exon of the predicted gene C39E6.1. We sequenced the corresponding cDNA (kindly sent by Yuji Kohara), and identified an ORF of about 500 amino acids which has a potential signal sequence at its N-terminus, is cysteine rich, and contains an RGD motif. In other proteins RGD motifs have been shown to interact with integrins. We have identified lesions associated with 5 other alleles of
lon-2 (kindly sent to us from the MRC collection). The canonical allele
e678 deletes exons encoding approximately the first 400 amino acids of LON-2; it is predicted to be a molecular null. The allele
e955 is a rearrangement of the 3' third of the
lon-2 ORF;
e405 and
e434 introduce premature stop codons;
e2140 changes one of the many cysteines in LON-2 to a tyrosine. We are currently examining whether excessive
lon-2 activity phenocopies
daf-4(lf) mutants. This would provide more compelling evidence that
lon-2 regulates TGF-b signalling. Extracellular LON-2 might regulate DAF-4 signalling by forming heterodimers with an activating ligand for DAF-4, or by restricting access of the DAF-4 ligand to the receptor.