Microtubule-Dependent Subcellular Redistribution of the Transcriptional Coactivator p/CIP

Abstract

The transcriptional coactivator p/CIP is a member of a family of nuclear receptor coactivator/steroid receptor coactivator (NCoA/SRC) proteins that mediate the transcriptional activities of nuclear hormone receptors. We have found that p/CIP is predominantly cytoplasmic in a large proportion of cells in various tissues of the developing mouse and in a number of established cell lines. In mouse embryonic fibroblasts, serum deprivation results in the redistribution of p/CIP to the cytoplasmic compartment and stimulation with growth factors or tumor-promoting phorbol esters promotes p/CIP shuttling into the nucleus. Cytoplasmic accumulation of p/CIP is also cell cycle dependent, occurring predominantly during the S and late M phases. Leptomycin B (LMB) treatment results in a marked nuclear accumulation, suggesting that p/CIP undergoes dynamic nuclear export as well as import. We have identified a strong nuclear import signal in the N terminus of p/CIP and two leucine-rich motifs in the C terminus that resemble CRM-1-dependent nuclear export sequences. When fused to green fluorescent protein, the nuclear export sequence region is cytoplasmic and is retained in the nucleus in an LMB-dependent manner. Disruption of the leucine-rich motifs prevents cytoplasmic accumulation. Furthermore, we demonstrate that cytoplasmic p/CIP associates with tubulin and that an intact microtubule network is required for intracellular shuttling of p/CIP. Immunoaffinity purification of p/CIP from nuclear and cytosolic extracts revealed that only nuclear p/CIP complexes possess histone acetyltransferase activity. Collectively, these results suggest that cellular compartmentalization of NCoA/SRC proteins could potentially regulate nuclear hormone receptor-mediated events as well as integrating signals in response to different environmental cues.Nuclear receptors (NRs) are a superfamily of structurally related proteins that function as ligand-regulated transcription factors. Members of this family include receptors for estrogen, glucocorticoids, nonsteroidal ligands such as thyroid hormone, and retinoic acid, as well as receptors that bind by-products of lipid metabolism such as fatty acids and prostaglandins. These receptors control a complex array of genes involved in many biological functions including cell proliferation and differentiation, metabolism and growth, morphogenesis, programmed cell death, and homeostasis. In the absence of hormone, some NRs such as the thyroid hormone receptor and retinoic acid receptor function as transcriptional repressors by interacting with corepressor proteins. Hormone binding results in a conformational change in the receptor that results in the release of corepressor proteins and in the recruitment of coactivator proteins (17).The nuclear receptor coactivator/steroid receptor coactivator (NCoA/SRC) proteins were among the first coactivators to be identified. This family includes steroid receptor coactivator 1 (SRC-1) (51) also designated nuclear receptor coactivator 1 (NCoA-1) (28); GRIP1 (25), also kno...

Role of the Nuclear Receptor Coactivator AIB1-Δ4 Splice Variant in the Control of Gene Transcription

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Redundant enhancement of mouse constitutive androstane receptor transactivation by p160 coactivator family members

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