Isoforms of photosystem II antenna proteins in different plant species revealed by liquid chromatography-electrospray ionization mass spectrometry

The high selectivity offered by reversed-phase highperformance liquid chrom atography on-line coupled to electrospray ionization mass spectrometry has been utilized to characterize the major and minor light-harvesting proteins of photosystem II (Lhcb). Isomeric forms of the proteins, revealed either on the basis of different hydrophobicity enabling their chromatographic sep aration or on the basis of different molecular masses identified within one single chromatographic peak, were readily identified in a number of monoco t and dicot species. The presence of several Lhcb1 isoforms (preferably in dicots) can explain the tendency of dicot Lhcb1 to form trimeric aggregates . The Lhcb1 molecular masses ranged from 24,680 to 25,014 among different s pecies, whereas within the same species, the isoforms differed by 14-280 ma ss units. All Lhcb1 proteins appear to be highly conserved among different species such that they belong to a single gene group that has several diffe rent gene family members. In all species examined, the number of isoforms c orresponded more or less to the genes cloned previously. Two isoforms of Lh cb3 were found in petunia and tomato. For Lhcb6, the most divergent of all light-harvesting proteins, the greatest number of isoforms was found in pet unia, tobacco, tomato, and rice. Lhcb2, Lhcb4, and Lhcb5 were present in on ly one form. The isoforms are assumed to play an important role in the adap tation of plants to environmental changes.