V. Citovsky et al., NUCLEAR IMPORT OF AGROBACTERIUM VIRD2 AND VIRE2 PROTEINS IN MAIZE ANDTOBACCO, Proceedings of the National Academy of Sciences of the United Statesof America, 91(8), 1994, pp. 3210-3214
Previously, we have shown that Agrobacterium-plant cell transferred DN
A (T-DNA) transport into the host cell nucleus is likely mediated by t
wo specific bacterial proteins, VirD2 and VirE2. Here, we used these p
roteins to study molecular pathways of nuclear import. First, the role
of VirE2 nuclear localization signals (NLSs) in the T-DNA transport p
athway was examined by using tobacco plants transgenic for deletion mu
tants of VirE2. In these plants, the virulence of wild-type Agrobacter
ium was reduced possibly by competition for the cellular nuclear impor
t machinery. Second, we analyzed the nuclear localization of VirE2 and
VirD2 in the nonhost monocot maize. Part of the known recalcitrance o
f monocots to transformation by Agrobacterium could be due to a potent
ial selectivity in nuclear import pathways in monocotyledonous and dic
otyledonous plants. Nuclear transport of VirD2 and VirE2 in maize leav
es and roots was compared to that in tobacco protoplasts and roots. Bo
th proteins accumulated in maize leaf and tobacco protoplast nuclei as
well as in nuclei of immature root cells. In contrast, VirD2 and VirE
2 expressed in mature roots of maize and tobacco remained cytoplasmic.
Point mutations of VirE2 nuclear localization signals, NSE 1 and NSE
2, also revealed that, in maize, the NSE 1 signal was mainly responsib
le for nuclear import; in contrast, both signals functioned independen
tly in tobacco protoplasts.