Molecular mechnisms of cell fusion during myogenic differentiation

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The cell fusion during skeletal muscle cell differentiation is considered to be induced by certain protein (s) with the fusion peptide. Intracellular signal transduction generated by this protein might regulate the cell fusion or some cellular events following the cell fusion. To elucidate the signal transduction pathway for cell fusion, we addressed the following three studies. (1) We examined the localization of the novel membrane protein ermelin, which contains a sequence homologous to the fusion peptide. Immunofluorescence microscopy in combination with epitope tagging and with an antibody to ermelin exhibited that the protein is present in the endoplasmic reticulum (ER). Immunoelectron microscopy located ermelin on the ER membrane. We are studying whether ermelin is involved in fusion of the ER membrane. (2) Meltrin alpha, which is reported to be participating in muscle cell fusion, contains in its cytoplasmic domain the consensus sequences for binding to the SH3 domain. The bindi … More nig experiments with bacterially expressed recombinant proteins and bovine brain extracts showed that the SH3 domains of Src, Yes, and Grb2 bound to the meltrin alpha cytoplasmic domain. Consequently, meltrin alpha is likely to be involved in some signal transduction for myogenic differentiation through the interaction with these proteins. (3) The novel small GTPases M-Ras and RhoD were cloned during investigation of the signal transduction for muscle cell fusion. We examined the cellularfunctions of M-Ras and RhoD by transfection of the cDNAs and microinjection of the recombinant proteins. M- Ras caused reorganization of the actin cytoskeleton including the microspike fromation and loss of the stress fibers. It induced dendritic cell morphology in fibroblasts and neurite outgrowth in PC12 cells. RhoD induced loss of the stress fibers and inhibited cell motility. It generated multinucleated cells by preventing cytokinesis. We are examining whether these GTPases are implicated in the signal transduction for cell fusion. Less