et al., 2001b). Moreover, CED-1 contains a NPXY motif and a YXXL motif (Zhou
et al., 2001b). These motifs are very common in the cytoplasmic domains of
transmembrane receptors, where they often direct phosphorylation of tyrosine
residues and the binding to adapter proteins. Consistent with CED-1’s being a
receptor, it is indeed localized to the cell surface, particularly on cell types that can
function as engulfing cells, and the presence of dying cells induces CED-1 clustering
around these cells (Figure 6) (Zhou et al., 2001b). CED-1 clustering is independent
of its cytoplasmic domain but dependent on the function of CED-7 (Zhou et al.,
2001b). It is not known what the actual substrate of the CED-1 receptor is. ced-7 is
the only engulfment gene known to play a role also in dying cells (Wu and Horvitz,
1998). CED-7 is similar to members of the ABC transporter superfamily that effect
ATP-dependent translocation of specific substrates across cellular membranes
(Figure 6) (Wu and Horvitz, 1998). Given these findings, two possible models for
CED-1 and CED-7 function have been suggested. First, CED-7 may facilitate
physical contact between dying cell and engulfing cell by exporting unknown
adhesion molecules to the cell surface of dying and engulfing cells. In a second
alternative model, CED-7 may function differently in dying and engulfing cells (Wu
and Horvitz, 1998). In engulfing cells, it might be required for the clustering of
CED-1. In dying cells, it might induce the exposure of apoptotic surface markers
Figure 6. Simplified model of corpse engulfment in C.elegans.
PROGRAMMED CELL DEATH IN C.ELEGANS 179