Kallmann Syndrome (KS) is a genetically heterogeneous disease combining anosmia and hypogonadism. The X-linked form of KS is caused by lesions in the KAL1 gene, which codes for a secreted molecule with similarities to neural cell adhesion molecules. In a modifier screen of an axon branching phenotype induced by kal-1, the C. elegans homologue, we obtained four strong suppressor mutants, three of which were previously mapped to genes coding for enzymes involved in heparan sulfate modifications (Bulow and Hobert, 2004). We describe here the characterization of the fourth strong suppressor mutation, the recessive allele ot20. Using a cosegregating temperature sensitive lethality we mapped ot20 to a 100kb interval on the proximal left arm of chromosome V. RNA interference and transgenic rescue experiments suggest that a gene named PAPS transporter (pst-1) is responsible for both the suppression of branching and the lethality. Furthermore, ot20 failed to complement all four available alleles of let-462, which mapped to a similar region and we have found mutations in both ot20 and alleles of let-462 within the pst-1 coding region. Sulfations of sugars, such as heparan sulfates (HS) or tyrosines require the universal sulfate donor PAPS (phosphoadenosyl-phosphosulfate) to be transported from the cytosol into the Golgi. Metazoan genomes encode two putative PAPS transporters (PAPST1 and PAPST2), which have been shown in vitro to preferentially transport PAPS across membranes. We have identified the C. elegans orthologs of PAPST1 and PAPST2 and named them pst-1 and pst-2, respectively. We show that pst-1 is essential for viability in C. elegans and can act non-autonomously to mediate its essential functions. pst-1 is contributed maternally and is specifically required during late embryonic and early larval stages. pst-1 null mutants arrest at the 3-fold stage of elongation probably due to aberrant muscle-hypodermis interactions. Additionally, pst-1 is required for specific aspects of nervous system development rather than formation of the major neuronal ganglia or fascicles. The neuronal defects correlate with reduced complexity of HS modification patterns as measured by direct biochemical analysis. Combining the biochemical results with epistatic analysis of HS modifying enzymes mutants we provide the first in vivo evidence that pst-1 is primarily involved in heparan sulfation. Our results suggest that pst-1 functions in metazoans to establish the complex HS modification patterns that are required for viability and the development of neuronal connectivity with high fidelity.