� 10 s, then spun at 16,000 *g* for 10 min. The resulting cell lysates were used for phosphatase activity assay. Absorbance values at 650 nm were recorded and normalized to the protein level (by Bradford method) of each sample. Relative PP1 activity is measured as the phosphatase activity (with 2 nM OA) minus the background phosphatase activity (with 1 µM OA).

Immunofluorescence and laser confocal imaging
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For PP1 WT and PP1 (T320A) immunostaining, primary hippocampal neurons were infected with recombinant Sindbis viruses encoding PP1α-my-HIS or PP1α-(T320A)-myc-HIS. The infected neurons were fixed 7 or 24 h after virus infection before neurons were incubated with blocking buffer (5% BSA and 5% normal goat serum in PBS) for 1 h. The neurons were then incubated with primary antibodies (Myc 9E10 mouse monoclonal antibody and synaptophysin rabbit polyclonal antibody) in blocking buffer for 1 h. After excess primary antibodies were washed out, the neurons were incubated for 1 h with Alexa 568--conjugated secondary anti--rabbit antibody (Invitrogen) and Alexa 488--conjugated secondary anti--mouse antibody (Invitrogen), both of which were diluted to 1:500 in the blocking buffer. The coverslips were mounted on slides with Fluoromount G (Electron Microscopy Sciences), an imaging medium. Immunoreactivity was acquired at room temperature by a confocal laser scanning microscope (LSM 510 Meta; Carl Zeiss) equipped with a Zeiss 63× oil immersion objective (NA 1.4) with SLM being the acquisition software. Final images were prepared using Adobe Photoshop software.

Immunogold electron microscopy (EM)
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Post-embedding immunogold labeling was based on established methods ([@bib45], [@bib46]). In brief, rats were perfused with 4% paraformaldehyde plus 0.5% glutaraldehyde, and sections were cryoprotected and frozen in a cryofixation system (EM CPC; Leica), and further processed and embedded in Lowicryl HM-20 resin using a freeze-substitution instr