Spartin recruits PKC-ζ via the PKC-ζ-interacting proteins ZIP1 and ZIP3 to lipid droplets

Protein kinase C-f interacting proteins (ZIP1-3) recruit the enzymatic activity of the atypical protein kinase C isoforms PKC-zeta/i or PKC-zeta to target proteins. In this study, we searched for binding partners of ZIP3 in the CNS and identified spartin, a multifunctional protein that is mutated in spastic paraplegia type 20. In transfected cells, spartin was present on the surface of lipid droplets (LD), whereas ZIP proteins appeared in intracellular speckles. In the presence of spartin, ZIP1 and ZIP3 were translocated to spartin-positive LD. This translocation was mediated by amino acids 196-393 of spartin that interacted with an N-terminal region of ZIP proteins. Furthermore, ZIP proteins interacted simultaneously with spartin and PKC-zeta, resulting in an enrichment of PKC-zeta on spartin/ZIP-labelled LD. Without spartin, neither ZIP proteins nor PKC-zeta were detected on LD. Interestingly, the presence of the spartin/ZIP/PKC-zeta complex increased LD size. This effect was most pronounced upon incorporation of the ZIP3 isoform into the trimer. Finally, we co-localized spartin, ZIP proteins and PKC-zeta in axon terminals of neurons in the mammalian retina. In summary, we describe spartin as new binding partner of the ZIP/PKC-zeta dimer that recruits PKC-zeta to LD and show that the expressed ZIP isoform regulates LD size.