vab-8 is required for most posteriorly directed cell and axon growth cone migrations (Wightman et al., Dev. 122:671(1996)). We cloned
vab-8 and found that it encodes a highly divergent kinesin-related protein. The N-terminal 330 amino acids of VAB-8 contains kinesin motor domain signature sequences. Moreover, the putative VAB-8 motor domain is predicted to have a nearly identical secondary structure to that of the recently crystallized human kinesin motor domain (Kull et al. Nature 380:550(1996)). Additionally, both kinesins and VAB-8 have long divergent C-terminal sequences with regions that can form coiled-coil dimerization domains. These similarities suggest that
vab-8 may function as a microtubule binding motor protein. However, VAB-8 is currently the most divergent kinesin-like molecule known.
vab-8 encodes two activities that can be defined by the effects
vab-8 mutations have on AVK axon outgrowth and CAN cell migration. Strong
vab-8 alleles affect both AVK outgrowth and CAN migration, whereas Unc
vab-8 alleles affect AVK outgrowth but not CAN migration. Molecularly
vab-8 encodes at least two proteins: a larger protein (VAB-8L) that includes the putative motor domain and a smaller protein (VAB-8S) that shares C-terminal sequences with VAB-8L but lacks the motor domain. Sequencing of
vab-8 alleles showed that strong alleles are frameshift and nonsense mutations that prematurely terminate both
vab-8 proteins, and that the Unc allele
ev411 is a splice donor site mutation that specifically disrupts VAB-8L. Furthermore, DNA clones encoding VAB-8S rescued the CAN migration but not AVK outgrowth defects of a strong
vab-8 mutant. Taken together, these results show that VAB-8L, but not VAB-8S, is necessary for AVK axon outgrowth, and that VAB-8S is sufficient for CAN cell migration. By mosaic analysis we determined that
vab-8 acts cell autonomously for CAN cell migration. Losses of an extrachromosomal array carrying the wild-type
vab-8 gene in the CAN cell lineage but not other lineages in a strong
vab-8 mutant background caused CAN cell migration defects. We propose that
vab-8 functions in the interpretation of a cue that directs cells and growth cones posteriorly. Antibodies raised against the protein region common to VAB-8L and VAB-8S stained body wall muscle cells of wild-type but not
vab-8 mutant animals. This staining was associated with dense body muscle attachment sites and was graded, with the strongest staining in the anterior of each cell. Because we were unable to detect
vab-8 immunoreactivity in cells in which
vab-8 is known to function, we expect that this muscle staining pattern represents a subset of the
vab-8 expression pattern. We cannot detect any muscle defects in
vab-8 mutants. Focal adhesions, which contain many of the molecular components found in dense bodies, play important roles in cell and axon growth cone migration. Thus the muscle expression pattern of
vab-8 may serve as a model for understanding the role of
vab-8 in posteriorly directed guidance.