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
Ka. Meadows et al., 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, Biochemistry, 34(5), 1995, pp. 1559-1574
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.