The mitochondrial protein hFis1 regulates mitochondrial fission in mammalian cells through an interaction with the dynamin-like protein DLP1.

The yeast protein Fis1p has been shown to participate in mitochondrial fission mediated by the dynamin-related protein Dnm1p. In mammalian cells, the dynamin-like protein DLP1/Drp1 functions as a mitochondrial fission protein, but the mechanisms by which DLP1/Drp1 and the mitochondrial membrane interact during the fission process are undefined. In this study, we have tested the role of a mammalian homologue of Fis1p, hFis1, and provided new and mechanistic information about the control of mitochondrial fission in mammalian cells. Through differential tagging and deletion experiments, we demonstrate that the intact C-terminal structure of hFis1 is essential for mitochondrial localization, whereas the N-terminal region of hFis1 is necessary for mitochondrial fission. Remarkably, an increased level of cellular hFis1 strongly promotes mitochondrial fission, resulting in an accumulation of fragmented mitochondria. Conversely, cell microinjection of hFis1 antibodies or treatment with hFis1 antisense oligonucleotides induces an elongated and collapsed mitochondrial morphology. Further, fluorescence resonance energy transfer and coimmunoprecipitation studies demonstrate that hFis1 interacts with DLP1. These results suggest that hFis1 participates in mitochondrial fission through an interaction that recruits DLP1 from the cytosol. We propose that hFis1 is a limiting factor in mitochondrial fission and that the number of hFis1 molecules on the mitochondrial surface determines fission frequency.

Dynamin 3 is a component of the postsynapse, where it interacts with mGluR5 and Homer.

The dynamins comprise a large family of mechanoenzymes known to participate in membrane modeling events. All three conventional dynamin genes (Dyn1, Dyn2, Dyn3) are expressed in mammalian brain and produce more than 27 different dynamin proteins as a result of alternative splicing. Past studies have suggested that Dyn1 participates in specialized neuronal functions such as rapid synaptic vesicle recycling, while Dyn2 may mediate the conventional clathrin-mediated uptake of surface receptors. Currently, the distribution, expression, and function of Dyn3 in neurons, or in any other cell type, are completely undefined. Here, we demonstrate that Dyn1 and Dyn3 localize differentially in the synapse. Dyn1 concentrates within the presynaptic compartment, while Dyn3 localizes to dendritic spine tips. Within the postsynaptic density (PSD), we found Dyn3, but not Dyn1, to be part of a biochemically isolated complex comprised of Homer and metabotropic glutamate receptors. Finally, although dominant-negative Dyn3 did not seem to inhibit receptor endocytosis, overexpression of a specific Dyn3 spliced variant in mature neurons caused a marked remodeling of dendritic spines. These data suggest that Dyn3 is a postsynaptic dynamin and, like its binding partner Homer, plays a significant role in dendritic spine morphogenesis and remodeling.