Worm Breeder's Gazette 13(3): 94 (June 1, 1994)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
We have been studying the lin-44 gene in order to understand how cell polarity becomes oriented during development. Mutations in lin-44 cause specific asymmetric cell divisions in the tail to be reversed in their polarities. Animals homozygous for either of two reduction-of-function lin-44 alleles or heterozygous for either allele over the deficiency hDf7 (which spans the lin-44 locus) display polarity reversals. We also have observed polarity reversals of cell divisions in deficiency homozygotes. These results indicate that the normal function of the lin-44 gene is to define the polarities of specific cell divisions (Herman and Horvitz, 1994, Development, 120: 1035-1047).
To learn how lin-44 controls cell polarity at the molecular level, the lin-44 locus was cloned by transformation rescue (Herman et al., WBG 13(2): 92). Microinjection of a 4.1-kb genomic clone rescues the lin-44 phenotype. Two similar cDNA clones that hybridize specifically to the 4.1-kb clone were isolated. Sequence analysis of the rescuing fragment as well as one of the cDNA clones reveals homology to the Wnt family of secretory glycoproteins. The putative lin-44 cDNA is 1278 bp in length and contains an open reading frame that could encode a protein of 350 amino acid residues. The open reading frame is followed by a 200-nucleotide 3' untranslated region that includes the polyadenylation signal AATAAA and a poly-A tail. The predicted lin-44 protein has several features in common with the Wnt family: 1 ) The size of the predicted lin-44 protein agrees with the average size of 350-380 amino acids for other Wnt proteins. 2) There appears to be an N-terminal hydrophobic signal peptide. 3) There is one prospective site for N-linked glycosylation (other Wnt proteins contain one or two such sites). 4) The predicted lin-44 protein contains 25 cysteine residues, and the positions of 22 of these are strongly conserved among members of the Wnt family. Comparison of the putative lin-44 protein to the other Wnt family members shows that LIN-44 is most similar to DWnt-2 (32.5% sequence identity) from Drosophila melanogaster and to Wnt-7 a(30.3% sequence identity) and 7b (30.7% sequence identity) from the mouse.
That lin-44 encodes a Wnt protein fits well with previous observations that suggest cell interactions may be involved in the orientation of cell polarity. The cells affected by lin-44 mutations are related by position and not by lineage history. In wild-type animals, the polarities of the F and U cells are known to depend upon the presence of the B cell, and in lin-44 mutants either all three of these cells have polarity reversals or all have normal polarity. In light of these observations, we envision a model in which a cell (or cells) in the tail expresses LIN-4 ,which functions to signal polarity information to neighboring cells. We are currently trying to identify those cells that express LIN-4 and those which require LIN-44 function.