Characterization of the cell wall and outer membrane of Rhodopseudomonas capsulata

Abstract

Sucrose density gradient centrifugation of cell envelopes of chemotropically grown cells of Rhodopseudomonas capsulata St. Louis (= ATCC 23782) resulted in the separation of a cytoplasmic membrane from a cell wall fraction (buoyant densities, 1.139 and 1.215 g/cm3, respectively). The cell wall fractions (untreated or Triton extracted) contained peptidoglycan- and lipopolysaccharide-specific components. Their neutral sugar content, mainly rhamnose and galactose, was high (250 and 100 μg/mg [dry weight] of material) due to a non-lipopolysaccharide polymer. The fatty acid content was low (≤60 μg/mg [dry weight] of material), and half of it was contributed by lipopolysaccharide (3-OH-C(10:0), C(12:1), and 3-oxo-C(14:0)). The predominant other fatty acid was C(18:1). An outer membrane fraction, obtained by lysozyme treatment of the Triton-extracted cell wall, showed essentially the same chemical composition except for almost complete removal of peptidoglycan. Saline extraction (0.9% NaCl, 37° C, 2 h) removed a lipopolysaccharide-protein(-phospholipid?) complex from whole cells of R. capsulata St. Louis. The polypeptide patterns of the cell wall and outer membrane as revealed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis comprised 20 to 25 different polypeptides (most of them very faint) and were dominated by a single, heat-modifiable major protein (M(r) 69,000 after solubilization below 60°C; M(r) 33,000 at temperatures above 70°C).