HA95 is a chromatin-associated protein that interfaces the nuclear envelope (NE) and chromatin. We report an interaction between HA95 and the inner nuclear membrane protein lamina-associated polypeptide (LAP) 2β, and a role of this association in initiation of DNA replication. Precipitation of GST–LAP2β fusion proteins and overlays of immobilized HA95 indicate that a first HA95-binding region lies within amino acids 137–242 of LAP2β. A second domain sufficient to bind HA95 colocalizes with the lamin B–binding domain of LAP2β at residues 299–373. HA95–LAP2β interaction is not required for NE formation. However, disruption of the association of HA95 with the NH2-terminal HA95-binding domain of LAP2β abolishes the initiation, but not elongation, of DNA replication in purified G1 phase nuclei incubated in S-phase extract. Inhibition of replication initiation correlates with proteasome-mediated proteolysis of Cdc6, a component of the prereplication complex. Rescue of Cdc6 degradation with proteasome inhibitors restores replication. We propose that an interaction of LAP2β, or LAP2 proteins, with HA95 is involved in the control of initiation of DNA replication.

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In several cases of immunodeficiency and autoimmunity, the dysfunctional immune system is associated with either hypo- or hyperactive T and B cells. In autoimmune conditions such as systemic lupus erythematosus (SLE) and immunodeficiencies such as acquired immunodeficiency syndrome (AIDS), it has been demonstrated that the regulatory effect of the signaling pathway of cyclic 3', 5' adenosine monophosphate (cAMP) and cAMP-dependent protein kinase (PKA) is abrogated. PKA is well-known as a key regulator of immune responses in that it inhibits both early and late phases of antigen induced T and B cell activation. Here we will discuss a potential useful strategy for therapeutic interventions of dysfunctional T cells associated with SLE and HIV by modulation of the cAMP-PKA pathway. Therefore, we will describe the components and architecture of the cAMP-PKA signaling pathway in T cells in order to point out one or several steps which potentially may serve as targets for therapeutic intervention.

Cyclic AMP‐dependent protein kinase (PKA) is a holoenzyme that consists of a regulatory (R) subunit dimer and two catalytic (C) subunits that are released upon stimulation by cAMP. Immunoblotting and immunoprecipitation of T‐cell protein extracts, immunofluorescence of permeabilized T cells and RT/PCR of T‐cell RNA using C subunit‐specific primers revealed expression of two catalytically active PKA C subunits Cα1 (40 kDa) and Cβ2 (47 kDa) in these cells. Anti‐RIα and Anti‐RIIα immunoprecipitations demonstrated that both Cα1 and Cβ2 associate with RIα and RIIα to form PKAI and PKAII holoenzymes. Moreover, Anti‐Cβ2 immunoprecipitation revealed that Cα1 coimmunoprecipitates with Cβ2. Addition of 8‐CPT‐cAMP which disrupts the PKA holoenzyme, released Cα1 but not Cβ2 from the Anti‐Cβ2 precipitate, indicating that Cβ2 and Cα1 form part of the same holoenzyme. Our results demonstrate for the first time that various C subunits may colocate on the same PKA holoenzyme to form novel cAMP‐responsive enzymes that may mediate specific effects of cAMP.

It is well documented that the β‐gene of the catalytic (C) subunit of protein kinase A encodes a number of splice variants. These splice variants are equipped with a variable N‐terminal end encoded by alternative use of several exons located 5′ to exon 2 in the human, bovine and mouse Cβ gene. In the present study, we demonstrate the expression of six novel human Cβ mRNAs that lack 99 bp due to loss of exon 4. The novel splice variants, designated CβΔ4, were identified in low amounts at the mRNA level in NTera2‐N cells. We developed a method to detect CβΔ4 mRNAs in various cells and demonstrated that these variants were expressed in human and Rhesus monkey brain. Transient expression and characterization of the CβΔ4 variants demonstrated that they are catalytically inactive both in vitro against typical protein kinase A substrates such as kemptide and histone, and in vivo against the cAMP‐responsive element binding protein. Furthermore, co‐expression of CβΔ4 with the regulatory subunit (R) followed by kinase activity assay with increasing concentrations of cAMP and immunoprecipitation with extensive washes with cAMP (1 mm) and immunoblotting demonstrated that the CβΔ4 variants associate with both RI and RII in a cAMP‐independent fashion. Expression of inactive C subunits which associate irreversibly with R may imply that CβΔ4 can modulate local cAMP effects in the brain by permanent association with R subunits even at saturating concentrations of cAMP.

BackgroundProtein kinase A type I (PKAI) and PKAII are expressed in most of the eukaryotic cells examined. PKA is a major receptor for cAMP and specificity is achieved partly through tissue-dependent expression and subcellular localization of subunits with different biochemical properties. In addition posttranslational modifications help fine tune PKA activity, distribution and interaction in the cell. In spite of this the functional significance of two forms of PKA in one cell has not been fully determined. Here we have tested the ability of PKAI and PKAII formed by expression of the regulatory (R) subunits RIα or RIIα in conjunction with Cα1 or Cβ2 to activate a co-transfected luciferace reporter gene, controlled by the cyclic AMP responsive element-binding protein (CREB) in vivo.ResultsWe show that PKAI when expressed at equal levels as PKAII was significantly (p < 0.01) more efficient in inducing Cre-luciferace activity at saturating concentrations of cAMP. This result was obtained regardless of catalytic subunit identity.ConclusionWe suggest that differential effects of PKAI and PKAII in inducing Cre-luciferace activity depend on R and not C subunit identity.