[RASMB] Time to reach equilibrium

[mmorris]

...so the exceptions indicate that a slow polymerisation includes an 
unfolding event exposing hydrophobic residues (at least as one 
possibility). Although, again, spectrin does not appear to fit that 
either. Self-association is of the indefinite (SEKIII) type with the 
protomer being the heterodimer. There are no nucleating steps or species 
as with actin or tubulin. The self-association is via the face of one 
helix fitting against the face of a pair of antiparallel helices to form 
a triple-helical bundle. Although electrostatic interactions appear to 
be important, hydrophobic interactions dominate the measured K values. 
An  'exposure' of these hydrophobic sites via unfolding (as John Philo 
suggests for his antibodies) does not appear to apply here. Yet, the 
self-association proceeds rapidly at 'high' temperatures (above about 25 
C) and extremely slowly at 'low' temperatures (below about 10 C). In 
velocity experiments, the dimer and tetramer boundaries can be 
completely separated in the range 4-20 C, and in fact can be used to 
measure quite accurately the K value for tetramerisation established by 
having previously incubated the sample at, say, 30 or 37 C, for an hour 
or so followed by rapid chilling and loading into the ultracentrifuge. 
Similarly, in low temperature native gels, run for about 48 h at 6 C, 
separate bands for species as large as the 22-mer can be observed. In 
velocity experiments above about 25 C, separate boundaries are no longer 
observed.

Finally, just to clarify, the self-association is completely reversible 
and is not complicated by disulfides. And the K values are not 
especially high (~10^6 M^-1).

Any ideas?

Yours, Michael

___________________________________________________________

Michael Morris
Senior Research Fellow
National Centre for Stem Cells and Tissue Repair
School of Molecular and Biomedical Science
University of Adelaide
AUSTRALIA 5005
Ph    : +61 8 8303 4551
Fax   : +61 8 8303 4348
e-mail: michael.morris at adelaide.edu.au
___________________________________________________________

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