Three genes,
unc-5,
unc-6, and
unc-40, have been shown to act in combination to guide circumferential migrations of cells or axons ( Hedgecock, E. M., Cubtti, J. G., and Hall, D. H. ,1990).
unc-5 affects dorsal migrations,
unc-40 affects primarily ventral migrations, and
unc-6 affects migrations in both directions. Previously, the sequence of the first 8 exons of the predicted
unc-6 mRNA was reported. These exons encode the N-terminus of the
unc-6 product and are homologous to domains Vl, V-1, V-2, and V-3 of laminin B chains. Because of its overall homology to the known B2 chain sequences,
unc-6 was assumed to be the C. elegans B2 chain homolog. However, northern analysis using a genomic probe from the
unc-6 coding region revealed a single 2.7 kb transcript in wildtype RNA. The
unc-6 mRNA is therefore much too small (sufficient for only 900 aa) to encode B chain domains I through Vl ( mouse and Drosophila B chains are between 1576 to 1768 aa). Furthermore, domains V-1 and V-3 of the
unc-6 laminin have B2 hallmarks, whereas domain V-2 and parts of domain Vl have B1 hallmarks. We are continuing the analysis of the
unc-6 gene, including finishing the genomic sequence. The evidence so far suggests that unc- 6 encodes a new type of guidance laminin which is directly involved in pioneer axon and mesodermal cell migrations.