Brain Actin-associated Protein Phosphatase 1 Holoenzymes Containing Spinophilin, Neurabin, and Selected Catalytic Subunit Isoforms

We previously characterized PP1bp134 and PP1bp175, two neuronal proteins that bind the protein phosphatase 1 catalytic subunit (PP1). Here we purify from rat brain actin-cytoskeletal extracts PP1 A holoenzymes selectively enriched in PP1γ 1 over PP1β isoforms and also containing PP1bp134 and PP1bp175. PP1bp134 and PP1bp175 were identified as the synapse-localized F-actin-binding proteins spinophilin (Allen, P. B., Ouimet, C. C., and Greengard, P. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 9956–9561; Satoh, A., Nakanishi, H., Obaishi, H., Wada, M., Takahashi, K., Satoh, K., Hirao, K., Nishioka, H., Hata, Y., Mizoguchi, A., and Takai, Y. (1998) J. Biol. Chem. 273, 3470–3475) and neurabin (Nakanishi, H., Obaishi, H., Satoh, A., Wada, M., Mandai, K., Satoh, K., Nishioka, H., Matsuura, Y., Mizoguchi, A., and Takai, Y. (1997) J. Cell Biol. 139, 951–961), respectively. Recombinant spinophilin and neurabin interacted with endogenous PP1 and also with each other when co-expressed in HEK293 cells. Spinophilin residues 427–470, or homologous neurabin residues 436–479, were sufficient to bind PP1 in gel overlay assays, and selectively bound PP1γ 1 from a mixture of brain protein phosphatase catalytic subunits; additional N- and C-terminal sequences were required for potent inhibition of PP1. Immunoprecipitation of spinophilin or neurabin from crude brain extracts selectively coprecipitated PP1γ 1 over PP1β. Moreover, immunoprecipitation of PP1γ 1 from brain extracts efficiently coprecipitated spinophilin and neurabin, whereas PP1β immunoprecipitation did not. Thus, PP1 A holoenzymes containing spinophilin and/or neurabin target specific neuronal PP1 isoforms, facilitating efficient regulation of synaptic phosphoproteins.