An insulin-like signaling pathway regulates diapause and life span in C. elegans . One component of this pathway,
age-1 , encodes a homolog of the
p110 catalytic subunit of mammalian phosphatidylinositol 3-kinase, PI(3)K (Morris, et al., 1996 Nature 382(536-539)). In vertebrates, a regulatory subunit known as
p85 or
p55 is required to activate
p110's catalytic activity in response to signaling from upstream tyrosine kinase receptors.
p85 and
p55 each contain two SH2 domains flanking an inter-SH2 domain which binds to and activates
p110 (Holt, et al., 1994 Mol Cell Biol 14(42-49)). Using the C. elegans genome sequence, we identified Cep55 , a homolog of vertebrate
p55. Genetic tests show that Cep55 acts in the same dauer-formation pathway as
age-1 and
daf-2 , an insulin receptor-like gene (Kimura, et al., 1997 Science 277(942-946)). Furthermore, recombinant CEP55 binds to a fragment of AGE-1 containing the predicted
p85/p55-binding domain. These results show that Cep55 encodes an
age-1 regulatory subunit. To further understand the cellular basis of insulin-like signaling in worms, the subcellular localizations of a full-length CEP55::GFP fusion and a truncated AGE-1(1-596)::GFP fusion were examined. Cep55::gfp and
age-1(1-596)::gfp are each expressed in many different, and sometimes overlapping, cells. Interestingly, in both cases, fluorescence can be found on intracellular vesicles in the soma of some neurons as well as diffusely in the cytoplasm. Mammalian PI(3)K has been suggested to play a role in regulating endosomal dynamics (Li, et al., 1995 PNAS 92(10207-10211)). We therefore propose that the CEP55- and AGE-1(1-596)-binding vesicles are endosomes. We have begun to examine whether
daf-2/age-1 insulin-like signaling in C. elegans shares conserved outputs with vertebrate insulin pathways. In particular, vertebrate insulin can stimulate cellular glucose uptake via PI(3)K by triggering plasma membrane targeting of vesicles containing the GLUT4 glucose transporter (Tanti, et al., 1996 JBC 271(25227-25232)). We are investigating whether
age-1 activity may similarly control glucose uptake in C. elegans .