Microbiol Monogr (2) J.M. Shively: Complex Intracellular Structures in Prokaryotes DOI 10.1007/7171_020/Published online: 20 May 2006 © Springer-Verlag Berlin Heidelberg 2006 Assembly and Disassembly of Phycobilisomes Noam Adir (u) · Monica Dines · Merav Klartag · Ailie McGregor · Meira Melamed-Frank Department of Chemistry, Technion, Israel Institute of Technology, Technion City, 32000 Haifa, Israel [email protected] 1Introduction................................... 48 2 Evolution and the Genetics of the Phycobilisome Antenna System ..... 52 3 Characteristics of the Phycobilisome Components .............. 55 3.1 TheBilinCofactors............................... 55 3.2 Allophycocyanin................................. 56 3.3 Phycocyanin................................... 58 3.4 Phycoerythrin.................................. 60 3.5 UnusualPhycobiliprotein............................ 61 3.6 LinkerProteins................................. 61 3.7 PhycobilisomeFunction............................. 64 4 Phycobilisome Assembly and Disassembly .................. 65 4.1 MonomerAssembly............................... 65 4.2 Assembly of the (αβ)3 Trimer......................... 67 4.3 (αβ)6 HexamerandRodAssembly....................... 67 4.4 Rod-coreAssociationandthePhycobilisomeStructure........... 68 4.5 PhycobilisomeDisassembly........................... 70 References ....................................... 72 Abstract The process of photosynthesis is initiated by the absorption of light energy by large arrays of pigments bound in an ordered fashion within protein complexes called antennas. These antennas transfer the absorbed energy at almost 100%efficiencytothe reaction centers that perform the photochemical electron transfer reactions required for the conversion of the light energy into useful and storable chemical energy. In prokary- otic cyanobacteria, eukaryotic red algae and cyanelles, the major antenna complex is called the phycobilisome, an extremely large (3–7 MDa) multi subunit complex found on the stromal side of the thylakoid membrane. Phycobilisomes are assembled in an ordered sequence from similarly structured units that covalently bind a variety of linear tetrapyrolle pigments called bilins. Phycobilisomes have a broad cross-section of absorp- tion (500–680 nm) and mainly transfer the absorbed energy to photosystem II. They can, however, function as an antenna of photosystem I, and their composition can be altered as a result of changes in the environmental light quality. The phycobilisome is structurally and functionally different from other classes of photosynthetic antenna complexes. In this review, we will describe the important structural and functional characteristics of the phycobilisome complex and its components, especially with respect to its assembly and disassembly. 48 N. Adir et al. Abbreviations APC allophycocyanin CCA complementary chromatic adaptation LHC light harvesting complex PB(s) phycobilin(s) PBS(s) phycobilisome(s) PBP(s) phycobiliprotein(s) PC phycocyanin PCB phycocyanobilin PE phycoerythrin PEB phycoerythrobilin PUB phycourobilin PXB phycoviobilin PSI photosystem I PSII photosystem II RC(s) reaction center(s) TEM transmission electron microscopy 1 Introduction A major priority of any photosynthetic organism is the efficient absorption of the light energy available in the specific environmental niche that it occupies. In search of efficient light absorption, different forms of light harvesting com- plex (LHC) antennas have evolved, with structural characteristics that lead to specific functional attributes (Adir 2005; Blankenship et al. 1995; Cogdell et al. 2004; Dekker and Boekema 2005; Frigaard et al. 2001, 2003; Glazer 1985; Huber 1989; Melkozernov and Blankenship 2005; Ting et al. 2002; Xiong and Bauer 2002). In all cases these antennas are composed of proteins that bind various pigment molecules in a fashion that affords both extremely efficient energy absorption and energy transfer to the photochemical reaction cen- ters (RCs). The sizes of these antennas vary from organism to organism, and the ratio of antenna pigments to RCs can be in the hundreds. The synthesis of a large excess of LHC pigments could be considered an energetic burden on the photosynthetic organism and is thus under tight regulation. The large sizes of antenna complexes indicate that it is imperative that the LHCs pro- vide enough excitation energy to the RCs in order to perform photosynthesis at the maximum efficiency and to avoid possibly deleterious back reactions (Adir et al. 2003). Many LHCs are made up of transmembrane proteins lo- cated within the thylakoid membranes adjacent to the RC (Ben-Shem et al. 2003; Cogdell et al. 2004; Dekker and Boekema 2005). In these cases, the two- dimensionality of the membrane limits the size of each LHC, and one finds multiple oligomeric forms in either condensed complexes (such as in LHCII and LHCI of plants), or in ring form (as found in purple non-sulfur bacteria). Assembly and Disassembly of Phycobilisomes 49 Two LHC types are located outside the membrane plane and are thus essen- tially less limited in their size. The largest of all LHCs is the chlorosome found in green sulfur bacteria (Blankenship et al. 1995; Frigaard et al. 2003). This structure is described in detail (Frigaard and Bryant 2006, in this volume) of this monograph. The second large non-membranous LHC is called the phyco- bilisome (PBS) (Fig. 1), found in cyanobacteria, red-algae and cyanelles (Adir 2005; Glazer 1989). In the following review we will describe the present state of knowledge on the structure and function of different PBS forms, with spe- cial emphasis on the mechanisms leading to the formation and disassembly of the PBS. The PBS successfully covers a wide range of excitation wavelengths using variations on a single structural principle. All PBS co-factor containing com- ponents (Table 1) are initially formed by a basic building block recognized in the literature as a monomer (Fig. 2). These monomers are in fact het- erodimers of two subunits (α and β) that are significantly homologous on both sequence and structural levels. Their structures and mode of associa- tion lead to a very even and highly symmetric structure; however, we will describe here how the differences in local environment, coupled with the manner of complex assembly, allows the PBS to efficiently perform energy ab- sorption and transfer. Each (αβ) monomer unit covalently binds up to three linear tetrapyrrole chromophore molecules called phycobilins (PBs) (Brown et al. 1990; MacColl 1998). The (αβ) monomers then self-assemble in a num- ber of apparently sequential steps (Fig. 3) into (i) trimeric (αβ)3 disks with adiameterof∼ 110 A˚,awidthof30 A˚ and an interior uneven aperture with adiameterof15–50 A˚; (ii) (αβ)6 hexameric double-disks; and finally into (iii) one of two major substructures, known as cores and rods. While both of these larger assemblies appear as stacks of disks aligned in different di- rections, it is recognized today that slight variations in the packing of these disks lead to markedly different properties. The number of core components Fig. 1 Electron micrograph of a Synechococcus lividus cell showing PBS particles attached to the thylakoid membranes (arrows). Magnification, ×60 000.Thefigurewaskindlypro- vided by Prof. E. Gantt and first appeared in Edwards, Gantt, J Cell Biol (1971), 50:896–900 50 N. Adir et al. Table 1 Phycobilisome protein subunits and pigments Protein Function Bilin Number Absorption Gene(s) component type 3 of bilins maximum (per monomer) (nm) 5 Allophycocyanin Major core PCB 2 652 apcA, apcB antenna protein Allophycocyanin Minor core PCB 1 671–679 apcD αB antenna protein Phycocyanin Rod antenna PCB 3 620 cpcA, cpcB protein 1 Phycocyanin612 Minor rod PCB 3 612 cpcA, cpcB antenna protein Phycoerythro- Rod antenna PCB 2 580–600 pccA, pccB cyanin protein PVB 1 Phycoerythrin Rod antenna PEB 4–5 4 495–565 cpeA, cpeB, protein PUB mpeA, mpeB 2 LR Internal rod – – – cpcC cpcH, cpcI, linker cpeC, cpeD, cpeE LC Internal core – – – apcC linker 2 LRC Rod-core – – – cpcD linker LCM Core-membrane PCB 1 671–679 apcE linker Phycoerythrin γ PE linker PEB 1 mpeC subunit and antenna PUB 1–3 1 PC612 has been identified only in PBS from T. vulcanus 2 AnumberofLR, LRC proteins can exist within each type of PBS, depending on species, and environmental conditions 3 PCB, phycocyanoblin; PVB, phycoviolobilin; PEB, phycoerythrobilin; PUB, phycouro- bilin 4 Different types of phycoerythrin contain 4–5 PEB, PUB bilins at different ratios 5 Absorption spectra of (αβ)3 trimeric forms (typically called cylinders) and rods is species dependent. One of the most common PBS forms consists of a tricylindrical core surrounded by six rods that form a hemidiscoidal semi-circle (Adir 2005; Glauser et al. 1992; Glazer 1989; MacColl 1998). Other PBS forms include two or five cylinders and dif- ferent numbers of rods (Ducret et al. 1998; Glauser et al. 1992). It is important to remember that while all of the different (αβ)3 disks appear to be similar, the association of disks to form either core cylinders or rods is architecturally different than the forces that provide for rod-core assembly. The original discovery and description of the PBS emanated from transmission electron microscopy (TEM) studies performed in the 1960s and 1970s (Fig. 1) (Bryant Assembly and Disassembly of Phycobilisomes 51 Fig. 2 Phycocyanin monomer. The phycocyanin monomer exemplifies all of the basic structural characteristics of PBPs. Panel A,theα and β subunits