Recruitment of Protein Phosphatase 1 to the Nuclear Envelope by A-Kinase Anchoring Protein AKAP149 Is a Prerequisite for Nuclear

Subcellular targeting of cAMP-dependent protein kinase (protein kinase A [PKA]) and of type 1 protein phosphatase (PP1) is believed to enhance the specificity of these enzymes. We report that in addition to anchoring PKA, A-kinase anchoring protein AKAP149 recruits PP1 at the nuclear envelope (NE) upon somatic nuclear reformation in vitro, and that PP1 targeting to the NE is a prerequisite for assembly of B-type lamins. AKAP149 is an integral membrane protein of the endoplasmic reticulum/NE network. The PP1-binding domain of AKAP149 was identified as [K.sup.153][GVLF.sup.157]. PP1 binds immobilized AKAP149 in vitro and coprecipitates with AKAP149 from purified NE extracts. Affinity isolation of PPI from solubilized NEs copurifies AKAP149. Upon reassembly of somatic nuclei in interphase extract, PP1 is targeted to the NE. Targeting is inhibited by a peptide containing the PP1-binding domain of AKAP149, abolished in nuclei assembled with membranes immunodepleted of AKAP149, and restored after reincorporation of AKAP149 into nuclear membranes. B-type lamins do not assemble into a lamina when NE targeting of PP1 is abolished, and is rescued upon recruitment of PP1 to the NE. We propose that kinase and phosphatase anchoring at the NE by AKAP149 plays in a role in modulating nuclear reassembly at the end of mitosis. Key words: AKAP * mitosis * nuclear envelope * protein kinase A * protein phosphatase 1