In vitro modulation of the interaction between HA95 and LAP2beta by cAMP signaling

Involvement of the catalytic subunit of protein kinase A and of HA95 in pre-mRNA splicing

Protein kinase A (PKA) is a holoenzyme consisting of two catalytic (C) subunits bound to a regulatory (R) subunit dimer. Stimulation by cAMP dissociates the holoenzyme and causes translocation to the nucleus of a fraction of the C subunit. Apart from transcription regulation, little is known about the function of the C subunit in the nucleus. In the present report, we show that both Calpha and Cbeta are localized to spots in the mammalian nucleus. Double immunofluorescence analysis of splicing factor SC35 with the C subunit indicated that these spots are splicing factor compartments (SFCs). Using the E1A in vivo splicing assay, we found that catalytically active C subunits regulate alternative splicing and phosphorylate several members of the SR-protein family of splicing factors in vitro. Furthermore, nuclear C subunits co-localize with the C subunit-binding protein homologous to AKAP95, HA95. HA95 also regulates E1A alternative splicing in vivo, apparently through its N-terminal domain. Localization of the C subunit to SFCs and the E1A splicing pattern were unaffected by cAMP stimulation. Our findings demonstrate that the nuclear PKA C subunit co-locates with HA95 in SFCs and regulates pre-mRNA splicing, possibly through a cAMP-independent mechanism.

Barrier-to-autointegration factor phosphorylation on Ser-4 regulates emerin binding to lamin A in vitro and emerin localization in vivo

Barrier-to-autointegration factor (BAF) is a conserved 10-kDa chromatin protein essential in proliferating cells. BAF dimers bind double-stranded DNA, histone H3, histone H1.1, lamin A, and transcription regulators, plus emerin and other LEM-domain nuclear proteins. Two-dimensional gel analysis showed that endogenous human and Xenopus BAF are posttranslationally modified by phosphorylation and potentially other modifications and that they are hyperphosphorylated during mitosis. The invariant Ser-4 residue on BAF is a major site of phosphorylation during both interphase and mitosis. In HeLa cells that overexpressed the phosphomimetic BAF missense mutant S4E, but not S4A, emerin mislocalized from the nuclear envelope, suggesting Ser-4-nonphosphorylated BAF normally promotes emerin localization at the nuclear envelope. Supporting this model, wild-type BAF but not mutant S4E enhanced emerin binding to lamin A in vitro. Thus, Ser-4-unphosphorylated BAF has a positive role in localizing emerin; this role may be disease relevant because loss or mislocalization of emerin causes Emery-Dreifuss muscular dystrophy. Our findings further suggest Ser-4 phosphorylation inhibits BAF binding to emerin and lamin A, and thereby weakens emerin-lamin interactions during both mitosis and interphase.

Dissociation of Emerin from Barrier-to-autointegration Factor Is Regulated through Mitotic Phosphorylation of Emerin in a Xenopus Egg Cell-free System

Emerin is the gene product of STA whose mutations cause Emery-Dreifuss muscular dystrophy. It is an inner nuclear membrane protein and phosphorylated in a cell cycle-dependent manner. However, the means of phosphorylation of emerin are poorly understood. We investigated the regulation mechanism for the binding of emerin to chromatin, focusing on its cell cycle-dependent phosphorylation in a Xenopus egg cell-free system. It was shown that emerin dissociates from chromatin depending on mitotic phosphorylation of the former, and this plays a critical role in the dissociation of emerin from barrier-to-autointegration factor (BAF). Then, we analyzed the mitotic phosphorylation sites of emerin. Emerin was strongly phosphorylated in an M-phase Xenopus egg cell-free system, and five phosphorylated sites, Ser49, Ser66, Thr67, Ser120, and Ser175, were identified on analysis of chymotryptic and tryptic emerin peptides using a phosphopeptide-concentrating system coupled with a Titansphere column, which specifically binds phosphopeptides, and tandem mass spectrometry sequencing. An in vitro binding assay involving an emerin S175A point mutant protein suggested that phosphorylation at Ser175 regulates the dissociation of emerin from BAF.