Immunoprecipitation and kinase activity experiments demonstrated that Ser-585 phosphorylation of the common beta chain (Bc) was required for activation of PI3K activity in response to IL-3 and co-precipitation of Bc, 14-3-3 zeta and the p85 subunit of Class 1A PI3 kinases (Guthridge et al. 2000). Subsequent experiments confirmed that Ser-585 phosphorylation and PI3K activation are required to promote cell survival in response to GM-CSF, but not for proliferation responses, and that this mechanism is independent of Bc tyrosine phosphorylation (Guthridge et al. 2004). This is one of two mechanisms described for the recruitment of PI3K to the IL-3/IL-5/GM-CSF receptors; the other involves Bc tyrosine-593 phosphorylation-mediated recruitment of SHC1, GRB2 and GAB2.