Analysis and Reconstitution of Phycobiliproteins: Methods for the Characterization of Bilin Attachment Reactions

Phycobiliproteins are a homologous family of light-harvesting accessory proteins present in cyanobacteria (
25
,
51
), red algae (
25
), cryptomonads (
36
,
52
), and some species of prochlorophytes (
41
,
48
). The blue, violet, red, or yellow colors of the phycobiliproteins are due to linear tetrapyrrole chromophores called bilins that are covalently attached at cysteine residues (
25
). These water-soluble proteins are composed of α and β subunits. The αβ monomers form (αβ)
3
trimers which further stack into (αβ)
6
hexamers. These discshaped trimers and hexamers can be stabilized or organized into larger structures by linker proteins. Through the association of several types of phycobiliproteins with these linker proteins [
69
), the large light-harvesting complex called the phycobili-some is formed (
51
,
63
). Cryptomonad phycobiliproteins have a different composition and structural organization and will not be discussed further in this chapter (for reviews on cryptomonad phycobiliproteins, see References
36
,
52
,
53
, and
73
).