ENZYMATIC AND CHEMICAL CLEAVAGE OF THE CORE LIGHT-HARVESTING POLYPEPTIDES OF PHOTOSYNTHETIC BACTERIA - DETERMINATION OF THE MINIMAL POLYPEPTIDE SIZE AND STRUCTURE REQUIRED FOR SUBUNIT AND LIGHT-HARVESTING COMPLEX-FORMATION

To ascertain the minimal structural requirements for formation of the subunit and core light-harvesting complex (LH1), the alpha- and beta-p olypeptides of the LH1 from three purple photosynthetic bacteria were enzymatically or chemically truncated or modified. These polypeptides were then used in reconstitution experiments with bacteriochlorophyll a (BChla), and the formation of subunit and LH1 complexes was evaluate d using absorbance and circular dichroism spectroscopies. Truncation o r modification outside of the conserved core sequence region of the po lypeptides had no effect on subunit or LH1 formation. However, the ext ent of formation; and stability of the subunit and LH1 decreased as th e polypeptide was shortened inside the core region within the N-termin al domain. This behavior was suggested to be due to the loss of potent ial ion-pairing and/or hydrogen-bonding interactions between the polyp eptides. While the spectroscopic properties of the subunit complexes g enerated using truncated polypeptides were analogous to those obtained using native polypeptides, in some cases the resulting LH1 complex ab sorption was blue-shifted relative to the control. Thus, truncation wi thin the N-terminal domain may have long-range effects on the immediat e BChla binding environment, since the putative BChla binding site res ides near the C-terminal end of the polypeptides. It was also demonstr ated that the His located within the membrane-spanning domain on the N -terminal end of the beta-polypeptide is not participating in ligation of the BChla in the reconstituted subunit and therefore probably not in LH1.