Patched is a multipass membrane protein controlling cell fate and proliferation that was initially identified in Drosophila. In humans, hereditary mutations in PTCH are associated with Gorlins syndrome and sporadic mutations can lead to the development of basal cell carcinomas. It has been demonstrated biochemically that Patched is a receptor for the Hedgehog morphogen. It is postulated that Patched and the serpentine membrane protein Smoothened form a complex whereby Patched inhibits the activity of Smoothened; Hedgehog relieves this inhibition by binding to Patched. In turn, Smoothened activates the transcriptional regulator Ci to express TGF-b and Wnt family members. We have found that the C. elegans genome encodes 2 Patched homologues and a closely related pseudogene. However, there are no obvious Hedgehog or Smoothened homologues encoded by the genome sequence although a large family of Hedgehog-like proteins have been identified (Aspck et al.1999. Genome Res. 9: 909-923), some of which may have signalling properties. Moreover, the activity of TRA-1, the single C. elegans homologue of Ci, appears to have been usurped by the sex determination pathway. This apparent absence of many of the components of the Hedgehog/Patched signalling pathway in C. eleganshas led us to ask what role the Ce-ptc genes might play in the development of the worm. From an evolutionary standpoint, a study of the Ce-ptc genes might shed light on the ancestral roles of Patched proteins or perhaps uncover new functions. We have cloned and characterised the
ptc-1, -2, and -3 genes. Both
ptc-1 and
ptc-3 are predicted to encode12-pass membrane proteins with topologies similar to Patched proteins identified in other organisms.
ptc-2 appears to be a pseudogene that arose through a recent duplication of the
ptc-1 gene. RNAi and mutational deletion studies indicate that
ptc-1 is an essential gene. Animals lacking
ptc-1 activity are essentially sterile with multinucleate germ cells arising from a probable defect in germline cytokinesis. We have also found that when the membranes separating individual germ cell nuclei are absent, such as in
ptc-1 mutants, multiple nuclei acquire the ability to cycle through mitosis synchronously. Thus these membranes appear to play an essential role in maintaining autonomous domains within the germline syncytium. in situ hybridization analyses indicate that
ptc-1 expression appears to be confined to the germ line and its progenitors. Antibody studies using anti-PTC-1 show that PTC-1 protein is associated with the plasma membrane of all germ cells, except mature sperm. The implications of these results will be discussed. We have also found that
ptc-3 is an essential gene. When
ptc-3 activity is removed by RNAi or by mutational deletion, animals die late in embryogenesis prior to hatching. However, the absence of
ptc-3 does not appear to disrupt germline development; hence, the mutant phenotypes of
ptc-1 and
ptc-3 are distinct. These findings indicate that the Ce-PTC proteins may have Hh- and Smo-independent activities and raise the possibility that the control of ptc activity has diverged during evolution.