• Whitley Vistisen posted an update 4 months, 1 week ago

    Furthermore, a gene whose mutant protein causes redistribution of the developmentally important nanos product has been shown to be a myosinlike protein. Furthermore, in the developing brain the mRNA for MAP has been found to be present in dendrites but not in neuronal cell bodies; this would appear to be physiologically relevant since in neurones the MAP protein is localized primarily in dendrites.Recent work suggests that not only do cells exhibit a specific subcellular distribution of mRNA but in addition their ribosomes appear to be organized in a spatial manner.This is evident for example in skeletal muscle where immunohistochemistry using anti S antibodies has shown ribosomal material to be associated with the myofibrils in addition to being present in the subsarcolemmal cytoplasm. These authors postulated that such ribosomes are involved in the synthesis of the myofibrillar proteins and this is supported by the observation that, in myoblasts, nascent myosin peptide chains being synthesized on polysomes are largely recovered in the cell matrix and released by pretreatment with cytochalasin D. Furthermore, these latter authors pointed out that translation of myosin mRNA on the cytoskeleton may allow for the site of translation to dictate where the myosin heavy chains are to be inserted into the cytoskeleton or myofibril.A second example of such spatial organization is found in the neurone where polysomes have been found in dendrites but not in axons and this would appear to be related to recent observations that have demonstrated transport of RNA from the cell body into the dendrites but not the axons of the hippocampal neurones; the cytoskeleton appears involved in this transport since the radiolabelled RNA was visualized in a detergentinsoluble cytoskeletal fraction.These two sets of observations are important examples of the spatial organization of ribosomes within cells and in both cases it appears to depend on some form of association with structural filaments.Cells must therefore possess a mechanism for the organization of ribosomes so that they become associated with cytoskeletal microfilaments, myofibrils or dendritic structures.The spatial organization of ribosomes is at present best explained by assuming that the cytoskeletalbound or myofibrillar ribosomes are involved in the synthesis of specific proteins.Exogenous mRNA microinjected into ooctyes becomes correctly localized, suggesting that the necessary directing information resides in the mRNA itself. This is supported by recent observations indicating that translocation of actin mRNA to the cell periphery is not dependent on the presence of nascent polypeptides or an association with intact ribosomes. areas of the cytoplasm.Detailed biochemical studies, using crosslinking techniques or in vitro translation with polysomecytoskeleton complexes, for example, will be necessary in order to provide such information.Clearly it will be important in future work to delineate both the relative importance and the different roles of these two filament systems.A scheme showing actincontaining microfilaments, although it would appear that how the two filament Icariin systems may participate in the organization sets of observations are best explained by the cytoskeleton being such as amphibian oocytes and skeletal muscle, and secondly the alterations in protein synthesis.A major priority in this field is to determine to what extent CBP synthesize a specific set of proteins.