Isolation and Characterization of the Outer Membrane Proteins of Azospirillum brasilense

SUMMARY: The outer membrane of Azospirillum brasilense was isolated from the total membrane fraction by sucrose density gradient centrifugation and by Sarkosyl extraction; both preparations showed an identical outer membrane protein profile in slab gels after electrophoresis under denaturing conditions. The profile showed a major 42 kDa protein constituting about 60% of the total outer membrane proteins. This major protein and a minor 40 kDa protein were tightly but non-covalently associated with peptidoglycan. In addition, a 23 kDa heat-modifiable protein, resistant to trypsin digestion, was detected in the outer membrane. Growth of A. brasilense under iron deficiency induced four additional high molecular mass proteins (87, 83, 78 and 72 kDa) in the outer membrane. A comparison of the outer membrane protein profiles of the three different Azospirillum species suggests that such profiles might be useful in taxonomic classification.