Allophycocyanin from Nostoc sp. phycobilisomes. Properties and amino acid sequence at the NH2 terminus of the alpha and beta subunits of allophycocyanins I, II, and III.

Allophycocyanin from Nostoc sp. phycobilisomes was separated into four spectrally distinct components designated allophycocyanin I, B, II, and III by adsorption chromatography on brushite columns. Allophycocyanins I and B had red-shifted fluorescence emission maxima, and on this basis, may function in transfer of excitation energy from phycobilisomes to chlorophyll a. Allophycyanins II and III, which together comprise 70% of the total allophycocyanin, have absorption maxima at 648 nm and 650 nm, respectively, and probably transfer excitation energy from phycocyanin to allophycocyanins I and B, in addition to serving a structural function. Allophycocyanin I was resolved into alpha, beta, and gamma subunits with apparent molecular weights of 18,000, 17,000, and 35,000, respectively, whereas allophycocyanin B was resolved into two subunits with apparent molecular weights of 16,100 and 15,300, using a modified Weber and Osborn gel electrophoresis system (Brown, A. S., and Troxler, R. F. (1977) Biochem. J. 163, 571-581). In the same gel system, allophycocyanins II and III were each resolved into alpha and beta subunits with apparent molecular weights of 18,000 and 17,000, respectively. The subunits of allophycocyanins I, II, and III were isolated by gel filtration and ion exchange chromatography and the amino acid compositions determined. Automated sequence analysis demonstrated that the first 30 amino acids at the NH2 terminus of alpha subunits, and the beta subunits, of allophycocyanins I to III were identical. alpha Subunits: Ser-Ile-Val-Thr-Lys-Ser-Ile-Val-Asn-Ala-Asp-Ala-Glu-Ala-Arg-Tyr-Leu-Ser-Pro-Gly -Glu-Leu-Asp-Arg-Ile-Lys-Ser-Phe-Val-Thr- beta Subunits: Ala-Gln-Asp-Ala-Ile-Thr-Ala-Val-Ile-Asn-Ala-Ala-Asp-Val-Gln-Gly-Lys-Tyr-Leu-Asp-Ala-Thr-Ala-Leu-Ser-Lys-Leu-Lys-Ala-Tyr- The gamma subunit of allophycocyanin I and both subunits of allophycocyanin B appeared to be blocked at the NH2 terminus, suggesting that the allophycocyanin B subunits may be different gene products than those of allophycocyanins I to III, or if the same, the subunits of allophycocyanin B undergo proteolytic modification after initial synthesis.