Protein kinase C μ is regulated by the multifunctional chaperon protein p32

We identified the multifunctional chaperon protein p32 as a protein kinase C (PKC)-binding protein interacting with PKCα, PKCζ, PKCδ, and PKCμ. We have analyzed the interaction of PKCμ with p32 in detail, and we show here in vivo association of PKCμ, as revealed from yeast two-hybrid analysis, precipitation assays using glutathione S-transferase fusion proteins, and reciprocal coimmunoprecipitation. In SKW 6.4 cells, PKCμ is constitutively associated with p32 at mitochondrial membranes, evident from colocalization with cytochrome c. p32 interacts with PKCμ in a compartment-specific manner, as it can be coimmunoprecipitated mainly from the particulate and not from the soluble fraction, despite the presence of p32 in both fractions. Although p32 binds to the kinase domain of PKCμ, it does not serve as a substrate. Interestingly, PKCμ-p32 immunocomplexes precipitated from the particulate fraction of two distinct cell lines, SKW 6.4 and 293T, show no detectable substrate phosphorylation. In support of a kinase regulatory function of p32, addition of p32 to in vitro kinase assays blocked, in a dose-dependent manner, aldolase but not autophosphorylation of PKCμ, suggesting a steric hindrance of substrate within the kinase domain. Together, these findings identify p32 as a novel, compartment-specific regulator of PKCμ kinase activity.