The Neuronal Actin-binding Proteins, Neurabin I and Neurabin II, Recruit Specific Isoforms of Protein Phosphatase-1 Catalytic Subunits

Neurabins are protein phosphatase-1 (PP1) targeting subunits that are highly concentrated in dendritic spines and post-synaptic densities. Immunoprecipitation of neurabin I and neurabin II/spinophilin from rat brain extracts sedimented PP1γ1 and PP1α but not PP1β. In vitro studies showed that recombinant peptides representing central regions of neurabins also preferentially bound PP1γ1 and PP1α from brain extracts and associated poorly with PP1β. Analysis of PP1 binding to chimeric neurabins suggested that sequences flanking a conserved PP1-binding motif altered their selectivity for PP1β and their activity as regulators of PP1 in vitro. Assays using recombinant PP1 catalytic subunits and a chimera of PP1 and protein phosphatase-2A indicated that the C-terminal sequences unique to the PP1 isoforms contributed to their recognition by neurabins. Collectively, the results from several different in vitro assays established the rank order of PP1 isoform selection by neurabins to be PP1γ1 > PP1α > PP1β. This PP1 isoform selectivity was confirmed by immunoprecipitation of neurabin I and II from brain extracts from wild type and mutant PP1γ null mice. In the absence of PP1γ1, both neurabins showed enhanced association with PP1α but not PP1β. These studies identified some of the structural determinants in PP1 and neurabins that together contribute to preferential targeting of PP1γ1 and PP1α to the mammalian synapse.