lin44 encodes a Wnt protein Michael A. Herman, Jocelyn E. Shaw, Larissa L. Vassilieva, and Robert K. Herman, Department of Genetics and Cell Biology, University of Minnesota, St. Paul MN 55108 We have been studying the
lin44 gene in order to understand how cell polarity becomes oriented dDg 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
lin44 alleles or heterozygous for either allele over the deficiency hDf7 (which spans the
lin44 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
lin44 gene is to define the polarities of specific cell divisions (Herman and Horvitz, 1994, development, 120: 1035-1047). To learn how
lin44 controls cell polarity at the molecular level, the
lin44 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 clanks reveals homology to the Wnt family of secretory glycoproteins. The putative
lin44 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
lin44 protein has several features in common with the Wnt family: 1) The size of the predicted
lin44 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
lin44 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
lin44 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 7a (30.3% sequence identity) and 7b (30.7% sequence identity) from the mouse. That
lin44 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
lin44 mutations arc 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 44, which functions to signal polarity information to neighbouring cells. We are currently trying to identify those cells that express LlN-44 and those which require LIN-44 function.