Crosslinking of Phycobiliproteins from the CyanobacteriumMastigocladus laminosuswith Bis-Imidates: Localization of an Intrasubunit and an Intersubunit Crosslink in C-Phycocyanin

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Abstract
The light-harvesting pigment-protein complexes allophycocyanin (AP), C-phycocyanin (PC) and phycoerythrocyanin (PEC) of the cyanobacterium Mastigocladus laminosus consist of .alpha.- and .beta.-subunits containing about 170 amino-acid residues each. These two subunits form an .alpha.,.beta.-monomer, three of which build up a disc-shaped trimer. In this study these phycobiliproteins were crosslinked with bisimidates. Various spacer lengths of the reagent and various aggregation states of the phycobiliprotein were tested. An intersubunit cross-link could be verified in all three phycobiliproteins. PC-trimers were crosslinked with the homobifunctional reagent dimethyl pimelimidate having a maximal crosslinking distance of 10 .ANG.. Two crosslinks could be identified: an intramonomer intersubunit crosslink with a yield of 48% and an intrasubunit crosslink within .alpha.PC (57%). These products were chemically and enzymatically fragmented and the small crosslinked peptides were isolated and then identified by amino-acid analysis. The following amino acids were crosslinked: .alpha.-Val 1 with .beta.-Ala 1 and .alpha.-Lys 62 with .alpha.-Lys 134. Both crosslinks could be localized within the known three-dimensional structure of PC.

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