The X proteins have been reported only in association with microtubules and with ribosomes in situ, in the normal central nervous system. In addition, T has been shown to be an integral component of paired helical rdaments, the principal constituent of the neurofibriflary tangles found in brains of patients with Alzheimer disease and of most aged individuals with Down syndrome (trisomy 21). We report here the localization of the well-characterized Tau-1 monoclonal antibody to the nucleolar organizer regions of the acrocentric chromosomes and to their interphase counterpart, the fibrillar component of the nucleolus, in human neuroblastoma cells. Similar localization to the nucleolar organizer regions was also observed in other human cell lines and in one monkey kidney cell line but was not seen in non-primate species. Immunochemically, we further demonstrate the existence of the entire Tmolecule in the isolated nuclei of neuroblastoma cells. Nuclear 7 proteins, like the 7 proteins of the paired helical filaments, cannot be extracted in standard SDS-containing electrophoresis sample buffer but require pretreatment with formic acid prior to immunoblot analysis. This work indicates that 7 may function in processes not directly associated with microtubules and that highly insoluble complexes of T may also play a role in normal cellular physiology.T proteins were first identified as a family of phosphoproteins that associate with microtubules in vivo and stimulate their assembly in vitro (1, 2). Recent evidence indicates that T proteins are an integral component of the paired helical filaments, the principal constituent of the neurofibrillary tangles characteristic ofAlzheimer disease or senile dementia of the Alzheimer type (3-7). The structure of the 7 gene and of the cDNAs cloned and sequenced from the expressed mRNAs reveals a tripartite protein composed of a variable N-terminal domain, a constant central domain, and a C-terminal, tubulin-binding domain (8,9). Hence, it appears that much of the observed electrophoretic heterogeneity is generated by alternative splicing of a single RNA transcript (8-10).Initially, r was reported to be restricted to axons within the central nervous system (11). A more widespread distribution was subsequently documented, indicating the presence of T along microtubules of both the axonal and somatodendritic compartments (12). Additionally, T was observed on ribosomes in neuronal somatodendritic compartments and in glial cells (12). We report here the localization of the Tau-1 monoclonal antibody (11) to the nucleolar organizer regions (NORs) of the acrocentric chromosomes (nos. 13, 14, 15, 21, and 22), in cultured human cells. Immunolocalization is also detected in the interphase counterpart of the NORs, the fibrillar component of nucleoli. Similar localization patterns are observed in cultured monkey kidney cells but are not present in non-primate cultured cell lines. The presence of T is biochemically documented in the isolated nuclei of two human neuroblastoma cell lines. ...

Abstract. We previously reported the presence of the microtubule-associated protein, tau in the nuclei of primate cells in culture. The present study confirms the existence of nuclear tau in two human neuroblastoma cells lines by indirect immunofluorescence and Western blot using mAbs to tau. Northern blot analysis of poly A ÷ mRNA detects a novel 2-kb tau transcript coexpressed with the 6-kb message in cultured human cells and human frontal cortex. PCR and cDNA sequencing demonstrate that the 2-kb message contains the entire tau coding region. Furthermore, actinomycin D transcription inhibition experiments indicate that the 2-kb message is not derived from the 6-kb message, but instead arises from the original tau transcript. One of the human neuroblastoma cell lines examined contains both nuclear and cytoplasmic tau as assayed by both Western blot and indirect immunofluorescence. Northern blot analysis of this cell line indicates that copious amounts of the 2-kb message are present while little of the 6-kb transcript is obvious. Immunofluorescence analysis of this cell line demonstrates that the cytoplasmic tau is not localized to microtubules. Together, these results indicate that the 2-kb tau message in humans may specify tau for nonmicrotubule functions in both the cytoplasm and the nucleus. We hypothesize that this is accomplished via a message targeting mechanism mediated by the untranslated regions of the tau messages.

Peripherin is a neural intermediate filament protein that is expressed in peripheral and enteric neurons, as well as in PC12 cells. A determination of the motile properties of peripherin has been undertaken in PC12 cells during different stages of neurite outgrowth. The results reveal that non-filamentous, non-membrane bound peripherin particles and short peripherin intermediate filaments, termed `squiggles', are transported at high speed throughout PC12 cell bodies, neurites and growth cones. These movements are bi-directional, and the majority require microtubules along with their associated molecular motors, conventional kinesin and cytoplasmic dynein. Our data demonstrate that peripherin particles and squiggles can move as components of a rapid transport system capable of delivering cytoskeletal subunits to the most distal regions of neurites over relatively short time periods.