Scapinin, a Putative Protein Phosphatase-1 Regulatory Subunit Associated with the Nuclear Nonchromatin Structure

Abstract

It is thought that the nuclear nonchromatin structures, such as the nuclear matrix and lamina, play regulatory roles in gene expression. In this study, we identified an insoluble protein that was associated with the chromatin-depleted nuclear structure of proliferating human leukemia HL-60 cells. Preparation of the chromatin-depleted nuclear structure, referred to as the nuclear matrix-intermediate filament scaffold (Fey, E., Krochmalnic, G., and Penman, S. (1986) J. Cell. Biol. 102, 1654 -1665), involved cell extraction using a series of buffers containing Triton X-100, DNase I, and 2 M NaCl. A yeast two-hybrid assay revealed that this protein bound to the catalytic subunit of protein phosphatase-1 (PP1). Furthermore, it inhibited PP1 activity in vitro. We therefore named it scapinin (scaffold-associated PP1 inhibiting protein). cDNA cloning revealed that scapinin had two splicing variants of 448 amino acids (scapinin-S) and 518 amino acids (scapinin-L). Scapinin was down-regulated by differentiation in HL-60 cells. These results suggest that scapinin is a putative regulatory subunit of PP1 and is involved in transformed or immature phenotypes of HL-60 cells. We also describe the presence of scapinin family proteins from worm to human.Protein phosphatase-1 (PP1) 1 is a major eukaryotic serine/ threonine protein phosphatase regulating diverse cellular processes such as muscle contraction, glycogen metabolism, RNA processing, and neuronal signaling (1-3). The catalytic subunit of PP1 binds to regulatory subunits that are critical for substrate specificity and spatial control of PP1 within the cell (4).It is thought that the nuclear nonchromatin structures, such as the nuclear matrix (NM) and nuclear lamina, are implicated not only in the spatial segregation of chromatins but also in the regulation of various nuclear processes associated with them (5-7). However, the regulatory mechanisms of the nuclear nonchromatin structure are not fully understood. The ␣-catalytic subunit of PP1 is associated with NM during interphase, and PP1 is a mitotic lamin phosphatase (8, 9). Recruitment of PP1 to the nuclear envelope by a PP1-binding protein, protein kinase A-anchoring protein AKAP-149, is a prerequisite for nuclear lamina assembly at the end of mitosis (10). Identification of PP1-binding proteins associated with the nuclear nonchromatin structure will provide insights into the regulatory roles of PP1 in the nuclear structure and gene expression. Aberrant nuclear structures are hallmarks of neoplastic transformation (11). Nuclear lamins alter during differentiation and transformation (12, 13). Many reports have described striking alterations in the NM proteins induced by transformation (14 -18). The mechanisms and physiological importance of this phenomenon are not fully understood, and identification of the alterations may provide more insight into differentiation and transformation.HL-60 cells have been used as an in vitro model of differentiation because various compounds can cause differentiation of these cells into ...

Scapinin-induced Inhibition of Axon Elongation Is Attenuated by Phosphorylation and Translocation to the Cytoplasm

“…Differing reports have described nuclear or cytoplasmic localization of scapinin in various transformed cell lines (1,4). Here we have shown that endogenous scapinin is present in the cytoplasm, nucleus, axon shaft, and growth cone of primary cortical neurons.…”

“…Interestingly, three putative nuclear localization sequences have been proposed by Sagara et al (1), with the second of these located adjacent to the Ser-277 phosphorylation site ( 277 pSPKKR 281 ; putative nuclear localization sequence is underlined). The relationship between phosphorylation and this particular nuclear localization sequence is difficult to investigate directly because mutation of the proline or basic residues to disrupt the putative nuclear localization sequence would almost certainly interfere with phosphorylation by the Ser-277 kinase.…”

“…It was originally identified as a component of the nuclear matrix intermediate filament scaffold (1). Since then, it has been detected in both the nucleus and the cytoplasm of transformed cell lines (1,2,4,5). In adult mouse brain, scapinin mRNA is enriched in the cortex and hippocampus (5), which also display relatively high neuroplasticity.…”

“…1). Scapinin is also able to bind to PP1 at its C terminus (1,5). A point mutation (F515A) was introduced that inhibits the interaction between scapinin and PP1 (5).…”

“…There are three splice variants of scapinin in mammals encoding proteins of 559 (62.5 kDa), 518 (58.5 kDa), and 448 (51.5 kDa) amino acids (1), with isoform 2 (518 amino acids) being predominant in human brain (4). It was originally identified as a component of the nuclear matrix intermediate filament scaffold (1). Since then, it has been detected in both the nucleus and the cytoplasm of transformed cell lines (1,2,4,5).…”