I. Plonsky et J. Zimmerberg, THE INITIAL FUSION PORE INDUCED BY BACULOVIRUS GP64 IS LARGE AND FORMS QUICKLY, The Journal of cell biology, 135(6), 1996, pp. 1831-1839
The formation of the fusion pore is the first detectable event in memb
rane fusion (Zimmerberg, J., R. Blumenthal, D.P. Sarkar, M. Curran, an
d S.J. Morris. 1994, J. Cell Biol. 127:1885-1894). To date, fusion por
es measured in exocytosis and viral fusion have shared features that i
nclude reversible closure (flickering), highly fluctuating semistable
stages, and a lag time of at least several seconds between the trigger
ing and the pore opening. We investigated baculovirus GP64-induced Sf9
cell-cell fusion, triggered by external acid solution, using two diff
erent electrophysiological techniques: double whole-cell recording (fo
r high time resolution, model-independent measurements), and the more
conventional time-resolved admittance recordings. Both methods gave es
sentially the same results, thus validating the use of the admittance
measurements for fusion pore conductance calculations. Fusion was firs
t detected by abrupt pore formation with a wide distribution of initia
l conductance, centered around 1 nS. Often the initial fusion pore con
ductance was stable for many seconds. Fluctuations in semistable condu
ctances were much less than those of other fusion pores, The waiting r
ime distribution, measured between pH onset and initial pore appearanc
e, fits best to a model with many (similar to 19) independent elements
. Thus, unlike previously measured fusion pores, GP64-mediated pores d
o not flicker, can have large, stable initial pore conductances lastin
g up to a minute, and have typical lag times of < 1 s, These findings
are consistent with a barrel-shaped model of an initial fusion pore co
nsisting of five to eight GP64 trimers that is lined with lipid.