Fig. 1. Structure and expression of zig genes. (A) Schematic structure of the predicted ZIG proteins. Protein domain, signal sequence (SS), transmembrane (TM), and GPI-anchor- predictions were performed as described in Web fig. 1 and based on PCR-amplified cDNAs. ZIG proteins are solely defined by the presence of exclusively two Ig domains (hence "zig" for "zwei Ig"); otherwise the ZIG proteins are highly divergent among themselves both in primary sequence (Web fig. 2) and exon/intron organization [panel (B)], suggesting anancient origin of this gene family. Database searches reveal that other metazoan genomes also contain a variety of secreted and transmembrane 2-Ig domain-only proteins whose role in neural development has not been explored (Web fig. 1B). (B) Structure of zig-promoter::gfp fusion constructs and summary of expression patterns (21). (C) Transgenic animals expressing zig::gfp fusion constructs. Arrows in insets point to PVT (21). (D) Temporal dynamics of
zig-4::gfp expression from the integrated array otIs20. Note the absence of
zig-4::gfp expression in PVT at embryonic and early L1 stages (upper left panel). Similar temporal expression profiles in PVT are observed with chromosomally integrated
zig-1,
zig-2,
zig-3, and
zig-8::gfp reporter genes.
zig-5::gfp expression can first be observed at late (>threefold) stages of embryonic development. (E) Timeline of zig gene expression in relation to C. elegans development, timing of pioneer axon outgrowth (1) and
unc-6/netrin expression (3). Expression of zig genes in PVT persists in adults, yet becomes less penetrant.