Interactor-mediated Nuclear Translocation and Retention of Protein Phosphatase-1

Abstract

Protein Ser/Thr phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that controls numerous cellular processes by the dephosphorylation of key regulatory proteins. PP1 is expressed in various cellular compartments but is most abundant in the nucleus. We have examined the determinants for the nuclear localization of enhanced green fluorescent protein-tagged PP1 in COS1 cells. Our studies show that PP1␥ 1 does not contain a functional nuclear localization signal and that its nuclear accumulation does not require Sds22, which has previously been implicated in the nuclear accumulation of PP1 in yeast (Peggie, M. W., MacKelvie, S. H., Bloecher, A., Knatko, E. V., Tatchell ؊/؊ mouse embryos showed a nuclear hyperphosphorylation on threonine, consistent with a role for NIPP1 in the nuclear targeting and/or retention of PP1. Our data suggest that both the nuclear translocation and the nuclear retention of PP1 depend on its binding to interactors with an RVXF motif.Protein Ser/Thr phosphatase-1 (PP1) 1 is expressed in all eukaryotic cells and controls numerous cellular processes including metabolism, cell division, apoptosis, and protein synthesis (1-5). PP1 does not exist freely in the cell but is associated with a large variety of polypeptides that determine when and where the phosphatase acts. Currently, ϳ70 mammalian genes are already known to encode interactors of PP1 (4, 5). Some of these function as targeting subunits and bring PP1 in close proximity to its substrates. Others (also) modulate the activity and substrate specificity of PP1 or are themselves substrates for associated PP1. The available information suggests that proteins interact with PP1 via multiple short sequence motifs. These PP1-binding motifs can be shared among PP1 interactors, accounting for the ability of PP1 to form stable complexes with a large number of structurally unrelated proteins. The best characterized and most common PP1-binding motif conforms to the consensus sequence RKX 0 -1 VI{P}FW in which X denotes any residue and {P} any residue except Pro; it is often referred to as the RVXF motif (6). The RVXF motif binds to a hydrophobic channel near the C terminus of PP1 (7). The binding of the RVXF motif not only has a PP1 anchoring function but also promotes the interaction of secondary, lower affinity binding sites, often resulting in an altered activity and/or substrate specificity of PP1 (1).Mammalian genomes contain three genes that together encode four isoforms of PP1, namely PP1␣, PP1␤/␦, and the splice variants PP1␥ 1 and PP1␥ 2 (1, 5). These isoforms are ϳ90% identical at the protein level and differ mainly in their extremities. With the exception of PP1␥ 2 , which is only expressed in the testis and the brain, the mammalian PP1 isoforms are ubiquitously expressed. PP1␣, PP1␤/␦, and PP1␥ 1 show an overlapping but distinct subcellular localization (8). Within the nucleus PP1␣ is mainly associated with the nuclear matrix, PP1␤/␦ is enriched in the non-nucleolar chromatin fraction, and PP1␥ 1 is predominantly targeted to the nucleo...

Ikaros Stability and Pericentromeric Localization Are Regulated by Protein Phosphatase 1

“…In cells, PP1 exhibits both cytoplasmic and nuclear localization (48), and it has been detected in chromosome-associated protein complexes (49,50), suggesting that PP1 is involved in transcriptional control (50). It has been demonstrated that the NIPP1 and PNUTS proteins can target PP1 to the nucleus (51). Our data suggest that expression of Ikaros regulates subcellular localization of PP1 in 293T cells and that Ikaros and a fraction of PP1 colocalize at PC-HC in human lymphoma cells.…”

Regulation of protein phosphatase type 1 and cell cycle progression by PfLRR1, a novel leucine‐rich repeat protein of the human malaria parasite Plasmodium falciparum

“…It is also important to point out that mutational studies indicate that the LRR cap domain present in the yeast Sds22 is probably involved in nuclear targeting while the central repetitive domain interacts with PP1 (Stone et al, 1993). Nevertheless, Lesage et al showed that PP1 can be targeted independently from Sds22 (Lesage et al, 2004) and complexed to Sds22 in the nucleus (Dinischiotu et al, 1997).…”

Nuclear Targeting of Protein Phosphatase-1 by HIV-1 Tat Protein

“…The catalytic subunit of PP1 lacks a nuclear localization signal and thus requires a regulatory subunit, such as NIPP1 and PNUTS, for targeting to the nucleus (41). We hypothesize that Tat functions as a nuclear targeting subunit of PP1 to deliver PP1 to specific nuclear substrate(s) important for the activation of HIV-1 transcription.…”