Small nuclear RNA-protein complex anchors on the actin filaments in bovine lymphocyte nuclear matrix.

Abstract

ABSTRACT. When the nuclear matrix from bovine lymphocytes was digested by RNase-depleted trypsin, the bulk of the matrix proteins, except actin, were hydrolyzed. The digestion left rapidly sedimented spherical structures (trypsin-treated nuclear matrix), which mainly were composed of actin (Nakayasu, H. and K. Ueda. Exp. Cell Res. 143, 55-62, 1983). Almost all the small nuclear RNAs of the original nuclear matrix remained associated with these actin spheres after trypsin digestion.By sonication, the small nuclear RNPs (snRNPs) in both untreated and trypsin-treated nuclear matrices were solubilized in association with proteinous filaments of various size. The sedimentation pattern of these snRNP complexes was not changed by the digestion of the bulk of the proteins. The snRNP complex was adsorbed on rabbit muscle myosin-Sepharose then eluted by the addition of 5 mM ATP. We concluded that snRNPs are associated with actin filaments in the nuclear matrix of bovine lymphocytes.Nuclei of eucaryotic cells contain a fibrogranular complex, generally called the nuclear matrix (2,6,14). Except for its surface lamina, which consists mainly of lamin proteins (12,16,35), little has been reported on the meshwork of fibrous materials that traverse the nuclear interior. It is likely that this interior meshwork participates in such nuclear functions as DNA replication (9, 15, 31), RNA processing (18,21,23).Recently, we found that actin is a main component of the interior materials (27) in the bovine lymphocyte nuclear matrix. The existence of nuclear actin also has been reported in rat liver (9), Xenopus oocytes (8, 22), Physarum polycephalum (17), and dimethylsulfoxide-treated cells (11,34). The function of this nuclear actin, however, has yet to be determined.SnRNPs have been found in isolated nuclear matrices from rat liver (24), fibroblasts (37), chick oviduct cells (7) and bovine lymphocytes (26). In a previous paper (26), we demonstrated that bovine snRNPs could be dissociated both with a high salt buffer and a low salt buffer that contained ATP, calcium ion, EDTA and DTT and that their release was sensitive to magnesium ion. The conditions of release were closely related to the condition that causes the depolymerization of F-actin.

Nuclear Speckles

“…Although an underlying scaffold that would serve as a platform on which to organize IGCs has thus far not been identified (Sacco-Bubulya et al 2002), several proteins with possible structural roles in the nucleus, such as a population of lamin A (Jagatheesan et al 1999) and snRNP-associated actin (Nakayasu et al 1984), have been detected in nuclear speckles. However, another study failed to detect an alteration in nuclear speckles in LMNA-/-cells (Vecerova et al 2004).…”

“…Interestingly, Hutchinson et al (Hutchinson et al 2007) identified MALAT1 (metastasis-associated lung adenocarcinoma transcript 1), a long nuclear retained noncoding RNA to be enriched in nuclear speckles (reviewed in Wilusz et al 2009). Recent studies have implicated MALAT1 in the recruitment of SR splicing factors from nuclear speckles to sites of transcription (Bernard et al 2010) and in the regulation of alternative splicing by modulating SF2/ ASF phosphorylation .Although an underlying scaffold that would serve as a platform on which to organize IGCs has thus far not been identified (Sacco-Bubulya et al 2002), several proteins with possible structural roles in the nucleus, such as a population of lamin A (Jagatheesan et al 1999) and snRNP-associated actin (Nakayasu et al 1984), have been detected in nuclear speckles. However, another study failed to detect an alteration in nuclear speckles in LMNA-/-cells (Vecerova et al 2004).…”

Ultrastructural localization of actin in nuclear matrices from mouse leukemia L5178Y cells.

“…From the intranuclear distribution of protein, we roughly estimated that the nuclear matrix contains about 30 % of the nuclear actin. As described elsewhere (11,12), actin is resistant to trypsin; but, there was the possibility of actin's reactivity with the anti-actin antibody being decreased by some alteration during trypsin digestion. Western blots of trypsin-treated and untreated nuclear matrices revealed, however, that trypsin digestion affected neither the actin content in the nuclear matrix nor actin's reactivity with the antibody (data not shown).…”

“…Little is known about the function of this nuclear actin, but several recent reports have revealed the involvement of nuclear actin in the synthesis (6,15), processing (10,12) and transport (13) of nascent nuclear RNAs. Therefore, we need to know what the architecture of this actin in the nucleus or nuclear matrix is.…”

Signal Transduction and Actin in the Regulation of G1-Phase Progression

“…168 Actin and profilin were also associated with snRNPs in nuclear speckles. 166,169,170 Interestingly, phosphatidylinositol 4,5-bisphosphate accumulates in these bodies as well as the phosphatidylinositol phosphate kinases (PIPKs). 171 Moreover, the localization of both PIPKs and Ptdlns(4,5)P 2 to speckles was described to reorganize upon inhibition of mRNA transcription, implicating a function for PIP 2 in transcription.…”