InterPro domain: IPR004155

This short bi-helical repeat is related to HEAT repeats and is present in phycocyanobilin lyases and other proteins.

Cyanobacteria and red algae harvest light energy using macromolecular complexes known as phycobilisomes (PBS), peripherally attached to the photosynthetic membrane. The major components of PBS are the phycobiliproteins. These heterodimeric proteins are covalently attached to phycobilins: open-chain tetrapyrrole chromophores, which function as the photosynthetic light-harvesting pigments. Phycobiliproteins differ in sequence and in the nature and number of attached phycobilins to each of their subunits. These proteins include the lyase enzymes that specifically attach particular phycobilins to apophycobiliprotein subunits. The most comprehensively studied of these is the CpcE/Flyase P31967 , P31968 , which attaches phycocyanobilin (PCB) to the alpha subunit of apophycocyanin [ 1 ]. Similarly, MpeU/V attaches phycoerythrobilin to phycoerythrin II, while CpeY/Z is thought to be involved in phycoerythrobilin (PEB) attachment to phycoerythrin (PE) I (PEs I and II differ in sequence and in the number of attached molecules of PEB: PE I has five, PE II has six) [ 2 ].

All the reactions of the above lyases involve an apoprotein cysteine SH addition to a terminal delta 3,3'-double bond. Such a reaction is not possible in the case of phycoviolobilin (PVB), the phycobilin of alpha-phycoerythrocyanin (alpha-PEC). It is thought that in this case, PCB, not PVB, is first added to apo-alpha-PEC, and is then isomerized to PVB. The addition reaction has been shown to occur in the presence of either of the components of alpha-PEC-PVB lyase PecE or PecF (or both). The isomerisation reaction occurs only when both PecE and PecF components are present, i.e. the PecE/F phycobiliprotein lyase is also a phycobilin isomerase [ 3 ]. Another member of this family is the NblB protein, whose similarity to the phycobiliprotein lyases was previously noted [ 4 ]. This constitutively expressed protein is not known to have any lyase activity. It is thought to be involved in the coordination of PBS degradation with environmental nutrient limitation. It has been suggested that the similarity of NblB to the phycobiliprotein lyases is due to the ability to bind tetrapyrrole phycobilins via the common repeated motif [ 5 ].

This repeat is also found in proteins not related to the phycobilisomes, such as archaeal proteins that are essential for chemotaxis and phototaxis [ 6 ], epoxyqueuosine reductases [ 7 ] and deoxyhypusine hydroxylases [ 8 ].

1. Oligomeric structure, enzyme kinetics, and substrate specificity of the phycocyanin alpha subunit phycocyanobilin lyase. J. Biol. Chem. 269, 8686-94
2. A role for cpeYZ in cyanobacterial phycoerythrin biosynthesis. J. Bacteriol. 179, 998-1006
3. Novel activity of a phycobiliprotein lyase: both the attachment of phycocyanobilin and the isomerization to phycoviolobilin are catalyzed by the proteins PecE and PecF encoded by the phycoerythrocyanin operon. FEBS Lett. 469, 9-13
4. A polypeptide with similarity to phycocyanin alpha-subunit phycocyanobilin lyase involved in degradation of phycobilisomes. J. Bacteriol. 181, 610-7
5. [Acetylation of the histones of the cell nuclei in the rat liver in radiation lesion] Radiobiologiia 17, 431-3
6. Identification of Archaea-specific chemotaxis proteins which interact with the flagellar apparatus. BMC Microbiol. 9, 56
7. Discovery of epoxyqueuosine (oQ) reductase reveals parallels between halorespiration and tRNA modification. Proc. Natl. Acad. Sci. U.S.A. 108, 7368-72
8. Successful delayed bilateral renal revascularization during active phase of Takayasu's arteritis. J. Vasc. Surg. 27, 552-4