PP1 activity in lysates of cells expressing 6His-HA-PP1α. Cells were cultured for 24 hr either in the absence (closed diamonds) or presence (closed squares) of doxycycline. PP1 was assayed using the indicated amounts of lysate/assay. 5 nM okadaic acid was used to inhibit PP2A during the assay. 1 unit of PP1 activity releases 1 nmol of Pi/min at 30°C.