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An elegant appraoch, simpler than BiFC or FRET was recently published. 
The authors used TIRF imaging of single complexes and counted subunits 
based on stepwise bleaching of GFP-tagged subunits.

Nat Methods. 2007 Apr;4(4):319-21. Epub 2007 Mar 18.
Subunit counting in membrane-bound proteins.
Ulbrich MH, Isacoff EY.
The subunit number and stoichiometry of membrane-bound proteins are 
difficult to
determine without disrupting their membrane environment. Here we describe a
single-molecule technique for counting subunits of proteins in live cell
membranes by observing bleaching steps of GFP fused to a protein of 
interest.
After testing the method with proteins of known stoichiometry expressed in
Xenopus laevis oocytes, we resolved the composition of NMDA receptors 
composed of
NR1 and NR3 subunits.

Christophe


Rosemary White a écrit :
> Search the CONFOCAL archive at
> http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal
>
> Dear all,
>
> Yes, I'm taking the lazy way out - have been asked about using FRET and/or
> FLIM to determine whether a membrane protein forms tetramers in the
> membrane.  Both C- and N-termini are cytoplasmic, and we were wondering
> about either FRET pairs or BiFC to test this.  Anyone have a quick 
> reference
> to this they knowof off hand?
>
> thanks,
> Rosemary
>
> Dr Rosemary White               [log in to unmask]
> CSIRO Plant Industry            ph.     02-6246 5475
> GPO Box 1600                        fax.     02-6246 5334
> Canberra, ACT 2601
> Australia
>