| Abstract: | XpsD is an outer membrane lipoprotein, required for the secretion of extracellular enzymes by Xanthomonas campestris pv. campestris. Our previous studies indicated that when the xpsD gene was interrupted by transposon Tn5, extracellular enzymes were accumulated in the periplasm (Hu, N.-T., Huung, M.-N., Chiou, S.-J., Tang, F., Chiang, D.-C., Huang, H.-Y. and Wu, C.-Y. (1992) J. Bacteriol. 174, 2679-2687). In this study, we constructed a series of substitutions and deletion mutant xpsD genes to investigate the role of NH/sub 2/- or COOH-terminal halves of XpsD in protein secretory function. Among these secretion defective xpsD mutants, one group (encoded by pCD105, pYL4, pKdA6 and pKD2) caused secretion interference when co-expressed with wild type xpsD, but the other (encoded by pMH7, pKdPs and pKDT) did not. Cross-linking studies and gel filtration chromatography analysis indicated that wild type XpsD forms multimer in its native state. Similar gel filtration analysis of xpsD mutants revealed positive correlations between multimer formation and secretion interfering properties exerted by the mutant XpsD in the parental strain XC1701. Those mutant XpsDs (encoded by pCD105, pYL4, pKdA6 and pKD2) that caused secretion interference could form multimers, and those (encoded by pMH7, pKdPs and pKDT) that did not could not. Furthermore, gel filtration and anion exchange chromatography analyses indicated that wild type XpsD co-fractionated with XpsD.DELTA.29-428 (encoded by pCD105) or XpsD.DELTA.448-650 (encoded by pKdPs) but not with XpsD.DELTA.74-303 (encoded by pYL4) or XpsD.DELTA.553-759 (encoded by pKDT). Models were proposed to explain the protein secretion interfering properties of XpsD.DELTA.29-428 (encoded by pCD105), XpsD.DELTA.448-650 (encoded by pKdPs), XpsD.DELTA.74-303 (encoded by pYL4) and XpsD.DELTA.553-759 (encoded by pKDT). |
