CORRESPONDENCE OF APOPROTEINS OF LIGHT-HARVESTING CHLOROPHYLL-A/B COMPLEXES ASSOCIATED WITH PHOTOSYSTEM-I TO CAB GENES - EVIDENCE FOR A NOVEL TYPE-IV APOPROTEIN

Apoproteins of spinach and pea light-harvesting chlorophyll a/b complexes associated with photosystem I (LHCI) were identified by their chlorophyll fluorescence spectra and protein sequences. Spinach LHCI holocomplex consisted of four apoproteins of 25 kDa, 23 kDa, 21 kDa and 20.5 kDa. LHCI subcomplex isolated by sucrose density gradient centrifugation fluoresced at 680 nm with a shoulder around 700-710 nm at 77 K. It contained the 23 kDa protein of which the N-terminal sequence corresponded to Type II gene of LHCI. Another LHCI subcomplex isolated by gel electrophoresis emitted at 679 nm and contained the 25 kDa protein, of which the N-terminus was blocked. Its internal sequences were determined after protease treatment and found to be homologous to Type III gene of LHCI. An oligomeric subcomplex of LHCI isolated by gel electrophoresis emitted at 726 nm and consisted of the 21 kDa and 20.5 kDa apoproteins. N-terminal sequence of the 20.5 kDa component corresponded to the Type I gene of LHCI. The 21 kDa component did not have any clear homologue, but its N-terminal sequence was weakly but significantly homologous to all LHC components particularly to Type I LHCI among others. It was, thus, concluded that the 21 kDa protein is the fourth type of LHCI apoprotein. Similar sequence homology was found for pea LHCI apoproteins.