Allophycocyanin (AP) was isolated from extracts of the cyanobacterium Mastigocladus laminosus. A fraction enriched in AP-associated polypeptides with apparent molecular masses of 21 -23 kD a in SDS-PAGE, was isolated on a preparative scale and identified as a homologous mixture of C-terminal fragments of the core-membrane linker polypeptide Lcm. The complex (αAPβAP)3 ·21 -23 kD a was reconstituted and characterized by sucrose density gradient ultracentrifugation, absorption, fluorescence emission and circular dichroism spectroscopy. The 21 -23 kD a polypeptides were found to induce spectral changes in AP similar to those induced by the small core linker polypeptide Lc8.9. Possible functions of the complex in phycobilisomes are discussed.
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