The molecular characteristics of soluble extensin from tomato have bee
n investigated. An apparent molecular mass greater than 240 kDa has be
en previously observed with the shape-dependent method of gel-filtrati
on chromatography [Brownleader and Dey (1993) Planta (Berlin) 191, 457
-469]. Tomato extensin is a heavily glycosylated protein that does not
migrate into SDS/polyacrylamide gels. This shape-dependent behaviour
raises doubts about agreement between the observed apparent mass and t
he absolute value. The molecular mass measured with matrix-assisted la
ser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF
MS) was 72.3 kDa, with no evidence of any other species except a doubl
y charged ion. The sample was therefore considered to be monodisperse
under the conditions used. Electron microscopy of soluble extensin sho
wed the presence of particles 40-50 nm in length and 2.0-2.5 nm in wid
th. A minority of these particles showed a central 'kink'. A number of
smaller and generally wider particles (20 nm x 2-4 nm) were considere
d to be folded monomers and larger particles were thought to be dimers
. Sedimentation analysis showed that extensin exists in a rapid monome
r-dimer equilibrium in the concentration range and buffer used. Sedime
ntation equilibrium data gave a K-d of 8.5 mu M and sedimentation velo
city data generated a K-d between 1 and 10 mu M. The concentration dep
endence of the measured sedimentation coefficient was used, together w
ith hydrodynamic bead modelling, to define plausible shapes for monome
r and dimer. This suggests that monomeric extensin is an elongated rod
of length 40 nm and width 2 nm, which forms staggered dimers of avera
ge length 50 nm and width 3 nm. Extensin is an integral component of t
he primary cell wall. The physical characteristics (size, shape and fo
rm) of the rod-like extensin have been evaluated in this paper so that
the role that extensin plays in primary cell wall architecture and du
ring plant disease resistance can be more fully understood.