GTPase activity of dynamin and resulting conformation change are essential for endocytosis

Dynamin family of mechanoenzymes

The dynamin family of proteins is continually growing, and in recent years members have been localized to areas of mitochondrial fission, plant phragmoplasts and chloroplasts, and viral ribonucleoprotein complexes. All the dynamin-like proteins examined to-date appear to assemble into oligomers, such as rings or spirals; however, it remains to be determined if a global mechanism of action exists. Even the role of dynamin in vesicle formation remains controversial as to whether it behaves as a molecular switch or as a mechanochemical enzyme.

Dynamin-related protein Drp1 is required for mitochondrial division in mammalian cells

Mutations in the human dynamin-related protein Drp1 cause mitochondria to form perinuclear clusters. We show here that these mitochondrial clusters consist of highly interconnected mitochondrial tubules. The increased connectivity between mitochondria indicates that the balance between mitochondrial division and fusion is shifted toward fusion. Such a shift is consistent with a block in mitochondrial division. Immunofluorescence and subcellular fractionation show that endogenous Drp1 is localized to mitochondria, which is also consistent with a role in mitochondrial division. A direct role in mitochondrial division is suggested by time-lapse photography of transfected cells, in which green fluorescent protein fused to Drp1 is concentrated in spots that mark actual mitochondrial division events. We find that purified human Drp1 can self-assemble into multimeric ring-like structures with dimensions similar to those of dynamin multimers. The structural and functional similarities between dynamin and Drp1 suggest that Drp1 wraps around the constriction points of dividing mitochondria, analogous to dynamin collars at the necks of budding vesicles. We conclude that Drp1 contributes to mitochondrial division in mammalian cells.

The role of phosphoinositides in membrane transport

Phosphoinositides serve as intrinsic membrane signals that regulate intracellular membrane trafficking. Recently, phosphoinositides have been found to direct the localization and activity of effector proteins containing consensus sequence motifs such as FYVE, PH and ENTH domains. In addition, recent results show that regulated synthesis and turnover of phosphoinositides by membrane-associated phosphoinoside kinases and phosphatases spatially restrict the location of effectors critical for cellular transport processes, such as clathrin-mediated endocytosis, autophagy, phagocytosis, macropinocytosis and biosynthetic trafficking.

Endocytotic mechanisms in synapses

Nerve terminals are highly enriched in proteins needed for endocytosis. Although constitutive and ligand-stimulated endocytosis take place in nerve terminals, the primary type is compensatory endocytosis--the process by which a cell retrieves the additional membrane added to cell surface by a regulated secretory event. This process has been extensively characterized using electrophysiological techniques. Except for an unusual form of coupled exo- and endocytosis called kiss-and-run release, compensatory endocytosis appears to use basically the same clathrin-mediated mechanisms as the constitutive and ligand stimulated type. The remarkable speed and selectivity of compensatory endocytosis may be achieved by concentrating the machinery at specialized sites in the nerve terminal adjacent to exocytosis sites and by the use of neuronal isoforms of the proteins that mediate endocytosis.

Cycling of synaptic vesicles: how far? How fast!

Synaptic transmission is based on the regulated exocytotic fusion of synaptic vesicles filled with neurotransmitter. In order to sustain neurotransmitter release, these vesicles need to be recycled locally. Recent data suggest that two tracks for the cycling of synaptic vesicles coexist: a slow track in which vesicles fuse completely with the presynaptic plasma membrane, followed by clathrin-mediated recycling of the vesicular components, and a fast track that may correspond to the transient opening and closing of a fusion pore. In this review, we attempt to provide an overview of the components involved in both tracks of vesicle cycling, as well as to identify possible mechanistic links between these two pathways.