GTP binds to the GTPase domain of dynamin. It's hydrolysis is stimulated when dynamin oligomerises around the neck of a budding vesicle. From sequence homologies and crystal structures, 4 nucleotide binding sequences can be identified: G1 to G4. These sequences are highlighted in red in the structures below of a Dictyostelium dynamin and human GBP1. The links below explore different aspects of GTPase domain function.
|Structures and backgrounds to these proteins
What are the functions of G1 to G4?
What are the structural explanations for GTP binding mutants? and what are the implications?
What does Shibire tell us?
Can we account for the unique kinetics of dynamin? (for v-type allostery and other kinetics see Stowell et al 1999)
Summary of the GTPase domain: The core of this domain is structurally similar to ras and the GTP binding sequences can easily be identified by sequence comparions, but mutations do not necessarily have the same effects on GTP hydrolysis. In contrast to ras, dynamin binds and releases GTP and GDP without the need for accessory proteins, and GTP hydrolysis is stimulated by self-oligomerisation rather than by a GAP. It is clear that mutants in G1 (or the P-loop) should not bind GTP and their affinity for GDP may also be decreased depending on which residues are being mutated. T65A in G2 is predicted to have its effect predominantly on catalysis as this takes away the coordination of the catalytic water molecule and the cofactor Mg2+, but otherwise it is not predicted to disturb the structure. Mutations in G4 should leave the structure in a nucleotide free state.
In cells the T65A mutant of dynamin I fails to catalyse the vesicle scission reaction and instead the necks of budding vesicles continue to grow longer and longer with time. Thus
1. GTP hydrolysis is necessary for vesicle scission.
2. The speed of hydrolysis is also critical. This is consistent with a mechanochemical stretching action of dynamin (see poppase model) where the cooperative and speedy stretching of the vesicle neck will cause it to pop, and if this reaction is slowed, then the neck will simply grow longer.