SPECTRAL MODIFICATIONS OF BACTERIAL ANTENNA COMPLEXES BY LIMITED PROTEOLYSIS

The spectral effects of a number of endo- and exoproteases to the detergent-solubilized antenna complexes B802-858 (sphaeroides like) of strain Rhodopseudomonas acidophila 10050 and B802-824 of strain 7050 were analyzed. Upon protease digestion the time course of spectral modification between 250 and 900 nm was recorded. In addition, circular dichroism (CD) signals were measured between 450 and 950 nm. The endoprotease elastase as well as the exoprotease carboxypeptidase A (CP A) altered the B802-858 antenna complex, by changing the 858 nm band hypochromically (30-50%) with an additional small hypsochromic shift (3-6 nm). A combined carboxypeptidase digestion, e.g. CP A and CP B, yielded a further modification to a complex with absorption bands at 800 nm and approximately 840 nm (epsilon approximately 50% of the 800 nm band). By exposing the isolated B803-824 antenna complex to the different proteases no significant spectral change was observed. The B802-858 antenna complex, modified by CP A, exhibits a 60-70% decrease of the biphasic CD signal in the near infrared. The limited proteolysis experiments provide conclusive experimental evidence that the C-terminal domains of the antenna polypeptides alpha and beta contribute to the formation of (dimeric) bacteriochlorophyll (BChl) a molecules absorbing at around 850 nm. This agrees with the hypothesis, based on comparative amino acid sequence analysis, that in purple bacterial antenna complexes the structural requirements (specific amino acids in the vicinity of the BChl molecules, e.g. aromatic amino acids) for batho- and hyperchromicity of BChl molecules apparently reside to a considerable extent in the C-terminal portions of their antenna apoproteins.