delta-Protocadherins: unique structures and functions.

delta-Protocadherins constitute a group of cadherins characterized by several conserved motifs in their cytoplasmic domains. We present a phylogenetic analysis that further divides this group into delta1-protocadherins (comprising protocadherin-1, -7, -9 and -11 or -X/Y) and delta2-protocadherins (comprising protocadherin-8, -10, -17, -18 and -19). The delta-protocadherin genes, which are located on different chromosomes in man and mouse, have a similar gene structure. They are expressed as multiple splice forms, differing mostly in their cytoplasmic domains. Some delta-protocadherins were reported to mediate weak cell-cell adhesion in vitro and cell sorting in vivo. In addition, individual delta-protocadherins might play important roles in signaling pathways, as they bind to proteins such as TAF1/Set, protein phosphatase-1alpha and the Frizzled 7 receptor. The spatiotemporally restricted expression of delta-protocadherins in different tissues and species and the results of their functional analysis, mainly in Xenopus, suggest that they play multiple, tightly regulated roles in vertebrate development.