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Mitochondrial DNA (mtDNA) repair occurs in all eukaryotic organisms and is essential for the maintenance of mitochondrial function. Evidence from both humans and yeast suggests that mismatch repair is one of the pathways that functions in overall mtDNA stability. In the mitochondria of the yeast Saccharomyces cerevisiae, the presence of a homologue to the bacterial MutS mismatch repair protein, MSH1, has long been known to be essential for mitochondrial function. The mechanisms for which it is essential are unclear, however. Here, we analyze the effects of two point mutations, msh1-F105A and msh1-G776D, both predicted to be defective in mismatch repair; and we show that they are both able to maintain partial mitochondrial function. Moreover, there are significant differences in the severity of mitochondrial disruption between the two mutants that suggest multiple roles for Msh1p in addition to mismatch repair. Our overall findings suggest that these additional predicted functions of Msh1p, including recombination surveillance and heteroduplex rejection, may be primarily responsible for its essential role in mtDNA stability.

The expression of a muscle-specific variant of microtubule-associated protein 4 (mMAP4) has been analyzed during myogenesis of C(2)C(12) cells using an isoform-specific antibody. MMAP4 localizes to microtubules (MTs) and is expressed prior to a very early morphogenetic event, the formation of mononucleate spindle-shaped cells. MMAP4 protein appears at about the same time as titin and coincident with Golgi reorganization, but antedates myosin expression. Misexpression of EGFP-mMAP4 in non-muscle and proliferating C(2)C(12) cells does not induce dramatic changes in MT organization or stability, nor in Golgi organization. Expression of full-length mMAP4 or of a truncated form lacking the MT-binding domain does not disrupt myotube formation or myofibrillogenesis. While previous antisense studies indicated that mMAP4 is necessary for normal myotube formation [Mangan and Olmsted, 1996: Development 122:771-781], these data indicate mMAP4 is not sufficient to induce the reorganization of MTs or the Golgi into patterns typical of muscle cells. Thus, with respect to MT organizing properties, this tissue-specific variant differs from related neuronal MAPs, MAP2, and tau, which induce neural-like changes in MT organization., (Copyright 2003 Wiley-Liss, Inc.)

A modified procedure is reported that enables microtubule assembly to occur in extracts of differentiated neuroblastoma cells. Of the proteins that co-assemble through five successive cycles, only two, with molecular weights of 215,000 and 71,000, are retained in constant ratio to tubulin. The 215,000-dalton protein is quantitatively sedimented with microtubules during later cycles of assembly, whereas the 71,000-dalton protein is distributed between assembling and nonassembling fractions. Similar procedures do not induce assembly for microtubules from nondifferentiated neuroblastoma cells. Two-dimensional gel analyses indicate that the differentiated cell extracts contain the 215,000-dalton protein. In contrast, gels of extracts from nondifferentiated cells show no protein in the equivalent region. These data suggest that the 215,000-dalton protein is a microtubule-associated protein that may play a role in microtubule-dependent neurite differentiation.