Fig. 1. Structure of cell surface HSPGs. (A) Predicted structure of the C. elegans syndecan SDN-1 and glypican GPN-1 proteins. Syndecan is a single-pass transmembrane protein with a short conserved cytoplasmic domain containing a PDZ-binding motif. HS side chains are attached to the extracellular domain of the core protein at conserved serine residues. The yellow box represents a typical tetrasaccharide linker region, which connects the HS chain to the serine. Glypican is attached to the cell surface by a GPI anchor. (B) Genomic structure of the sdn-1 locus on chromosome X. Boxes represent exons (blue, coding; black, non-coding) and red triangles indicate putative HS attachment sites. Two isoforms of the sdn-1 mRNA have been predicted to code for identical proteins. The null allele sdn-1(zh20) affects exons 1-5. The allele ok449 was previously shown to produce a truncated syndecan form lacking the two major HS attachment sites (Minniti et al., 2004). (C) Northern blot analysis of sdn-1. A single message of approximately 0.9 kb is detected in wild-type worms, whereas no message can be found in sdn-1(zh20) mutants. 18S and 28S ribosomal RNAs, as loading controls, are shown to the right.