Agriculture Reference
In-Depth Information
GNOM
BFA
Endosome
GTP
GDP
ARF
Coat
PIN1
TIBA
IAA
−
PIN1
Figure 1.4
Subcellular movement of PIN proteins. Schematic model to explain internalization of PIN
proteins upon BFA treatment. BFA blocks GNOM ARF-GEF responsible for activation of endosomal
ARF GTPases, which mediate recycling of PIN1 to the plasma membrane. Ongoing endocytosis is
BFA insensitive. AEIs such as TIBA interfere with both steps of PIN cycling.
BFA are guanine nucleotide exchange factors (GEFs), which activate small GTPases
of the ARF family - important regulators of vesicle budding. In
Arabidopsis
roots,
PIN1 protein is rapidly and reversibly internalized from the plasma membrane in re-
sponse to BFA (Fig. 1.4) (Geldner
et al
., 2001). This also occurs in the presence of a
protein synthesis inhibitor, thereby demonstrating that internalized PIN1 originated
from the plasma membrane and that PIN1 is rapidly cycling between the plasma
membrane and a 'BFA' compartment, recently characterized as an accumulation of
endosomes (Plate 1.1E) (Geldner
et al
., 2003). The action of drugs that disrupt the
structure of the cytoskeleton indicated that PIN1-containing vesicles are transported
predominantly along the actin cytoskeleton. However, in dividing cells, tubulin is
also required for correct PIN1 traffic (Geldner
et al
., 2001). The actin-dependent
recycling was also demonstrated for PIN3 (Friml
et al
., 2002a) and PIN2 (Grebe
et al
., 2003) proteins. The PIN recycling phenomenon has been further corrobo-
rated by electron microscopy studies, which detected PIN3 not only at the plasma
membrane but frequently also in intracellular vesicles (Friml
et al
., 2002a). BFA is
also known to rapidly interfere with auxin efflux and could phenocopy the effect of
AEIs (Delbarre
et al
., 1998; Morris & Robinson, 1998; Geldner
et al
., 2001). These
surprising findings can be hardly incorporated in the old static models, with influx
and efflux carrier complexes residing and functioning at the plasma membrane. The
