Intracellular structure and nucleocytoplasmic transport

Electron tomography reveals posttranscriptional binding of pre-mRNPs to specific fibers in the nucleoplasm

Using electron tomography, we have analyzed whether the Balbiani ring (BR) pre-mRNP particles in transit from the gene to the nuclear pore complex (NPC) are bound to any structure that could impair free diffusion through the nucleoplasm. We show that one-third of the BR particles are in contact with thin connecting fibers (CFs), which in some cases merge into large fibrogranular clusters. The CFs have a specific protein composition different from that of BR particles, as shown by immuno-EM. Moreover, we have identified hrp65 as one of the protein components of the CFs. The sequencing of hrp65 cDNA reveals similarities with hnRNP proteins and splicing factors. However, hrp65 is likely to have a different function because it does not bind to nascent pre-mRNA and is not part of the pre-mRNP itself. Taken together, our observations indicate that pre-mRNPs are not always freely diffusible in the nucleoplasm but interact with fibers of specific structure and composition, which implies that some of the posttranscriptional events that the pre-mRNPs undergo before reaching the NPC occur in a bound state.

Modification of cytoskeleton and prosome networks in relation to protein synthesis in influenza A virus-infected LLC-MK2 cells

Modifications of the cytoskeleton and protein synthesis were investigated in LLC-MK2 cells during infection by FPV/Ulster 73, an avian strain of influenza A virus. During infection, the cytoskeleton and the prosome networks undergo a dramatic reorganization, which seems to be at least temporally differentiated for each cytoskeletal system, i.e. microfilaments (MFs), microtubules (MTs), intermediate filaments (IFs). In order to evaluate the role of the three different cytoskeletal networks during FPV/Ulster infection, studies were carried out on cellular and virus-specific protein synthesis and viral production, using drugs which selectively affect individual cytoskeletal systems. Our data show that the perturbation of the IF system, but not that of the MFs or MTs, seems to have a strong inhibitory effect on virus production and cellular and viral protein synthesis. Furthermore, the dynamics of IFs and prosomes were investigated during viral infection and, at no time, dissociation of the prosome and IF networks was observed. Taken together, these results strongly support the idea that the interactions between the protein synthesis machinery, the cytoskeleton, and the prosomes are all affected by viral infection in a partially coordinated manner.

Prosome cytodistribution relative to desmin and actin filaments in dividing C2.7 myoblasts and during myotube formation in vitro

Prosomes constitute the multicatalytic proteinase (MCP) core of the 26S proteasomes, but were first observed as subcomplexes of untranslated mRNP; this suggests that they play a putative role in the control of protein biosynthesis in addition to their catabolic enzymatic function. In previous investigations it was shown that some prosomes colocalize with the intermediate filaments (IF) of the cytoskeleton, of the cytokeratin type in epithelial cells, and of the vimentin type in fibroblasts. Studies on adult rat muscle carried out with prosome-specific monoclonal antibodies (p-mAbs) have shown, surprisingly, that specific types of prosomes predominantly occupy a particular zone in between the M and the Z lines of the sarcomeric structure. The data presented here show that the subunit composition of prosomes changes when the dividing C2.7 myoblasts fuse into myotubes. We show furthermore that, in dividing C2.7 myoblasts, prosomes colocalize with the desmin network as well as with that of actin, in a distribution that changes with the subunit pattern of the prosomes investigated by individual p-mAbs. Surprisingly, when myogenic fusion is induced, specific types of prosomes move first to the nuclei; later on, they reappear in the cytoplasm. There, superimposing initially onto the reorganizing desmin filaments that run from one pole of the prefusion myoblast to the other, prosomes gradually colocalize with the actin fibers in the fusing myotubes, finally forming a "pearl on a string" pattern. These results are discussed in relation to parallel observations of prosome distribution between the actin and IF networks not only in epithelial cells but also in fusing muscle satellite cells, which made it possible to monitor the complete buildup of the sarcomeric structure.