Control of Mitochondrial Morphology Through Differential Interactions of Mitochondrial Fusion and Fission Proteins

Mitochondria in mammals are organized into tubular networks that undergo frequent shape change. Mitochondrial fission and fusion are the main components mediating the mitochondrial shape change. Perturbation of the fission/fusion balance is associated with many disease conditions. However, underlying mechanisms of the fission/fusion balance are not well understood. Mitochondrial fission in mammals requires the dynamin-like protein DLP1/Drp1 that is recruited to the mitochondrial surface, possibly through the membrane-anchored protein Fis1 or Mff. Additional dynamin-related GTPases, mitofusin (Mfn) and OPA1, are associated with the outer and inner mitochondrial membranes, respectively, and mediate fusion of the respective membranes. In this study, we found that two heptad-repeat regions (HR1 and HR2) of Mfn2 interact with each other, and that Mfn2 also interacts with the fission protein DLP1. The association of the two heptad-repeats of Mfn2 is fusion inhibitory whereas a positive role of the Mfn2/DLP1 interaction in mitochondrial fusion is suggested. Our results imply that the differential binding of Mfn2-HR1 to HR2 and DLP1 regulates mitochondrial fusion and that DLP1 may act as a regulatory factor for efficient execution of both fusion and fission of mitochondria

Rtn1p Is Involved in Structuring the Cortical Endoplasmic Reticulum

The endoplasmic reticulum (ER) contains both cisternal and reticular elements in one contiguous structure. We identified rtn1Δ in a systematic screen for yeast mutants with altered ER morphology. The ER in rtn1Δ cells is predominantly cisternal rather than reticular, yet the net surface area of ER is not significantly changed. Rtn1-green fluorescent protein (GFP) associates with the reticular ER at the cell cortex and with the tubules that connect the cortical ER to the nuclear envelope, but not with the nuclear envelope itself. Rtn1p overexpression also results in an altered ER structure. Rtn proteins are found on the ER in a wide range of eukaryotes and are defined by two membrane-spanning domains flanking a conserved hydrophilic loop. Our results suggest that Rtn proteins may direct the formation of reticulated ER. We independently identified Rtn1p in a proteomic screen for proteins associated with the exocyst vesicle tethering complex. The conserved hydophilic loop of Rtn1p binds to the exocyst subunit Sec6p. Overexpression of this loop results in a modest accumulation of secretory vesicles, suggesting impaired exocyst function. The interaction of Rtn1p with the exocyst at the bud tip may trigger the formation of a cortical ER network in yeast buds