VIRB1, A COMPONENT OF THE T-COMPLEX TRANSFER MACHINERY OF AGROBACTERIUM-TUMEFACIENS, IS PROCESSED TO A C-TERMINAL SECRETED PRODUCT, VIRB1

During genetic transformation of plant cells by Agrobacterium tumefaci ens, 11 VirB proteins and VirD4 are proposed to form a transmembrane b ridge to transfer a DNA-protein complex (T-complex) into the plant cyt oplasm, In this study, the localization of the first product of the vi rB operon, VirB1, was studied in detail, While full-length VirB1 local ized mostly to the inner membrane, an immunoreactive VirB1 product was found as soluble processed form, designated VirB1, Equal amounts of VirB1 could be detected in concentrated culture supernatants versus a ssociated with the cell, VirB1 was purified from the supernatant of v ir-induced cells by ammonium sulfate precipitation and Q-Sepharose chr omatography, Sequence analysis of the N terminus of VirB1 localized t he processing site after amino acid 172 of VirB1, Cell-associated VirB 1 was partly removed by vortexing, suggesting a loose association wit h the cell or active secretion, However, crosslinking and coimmunoprec ipitation show ed a close association of cell-bound VirB1 with the Vi rB9-VirB7 heterodimer, a membrane-associated component of the T-comple x transfer machinery, Homologies of the N-terminal part of VirB1 to ba cterial transglycosylases suggest that it may assist T-complex transfe r by local lysis of the bacterial cell wall, whereas the exposed local ization of the C-terminal processing product VirB1 predicts direct in teraction with the plant, Thus, VirB1 may be a bifunctional protein wh ere both parts have different functions in T-complex transfer from Agr obacterium to plant cells.