Structural Characterization of in Vitro and in Vivo Intermediates on the Loading Module of Microcystin Synthetase

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Abstract
The microcystin family of toxins is the most common cause of hepatotoxicity associated with water blooms of cyanobacterial genera. The biosynthetic assembly line producing the toxic cyclic peptide, microcystin, contains an adenylation–peptidyl carrier protein didomain (A–PCP) at the N-terminus of the initiator module McyG (295 kDa) that has been postulated to activate and load the starter unit phenylacetate for formation of the unusual aromatic β-amino acid residue, Adda, before subsequent extension. Characterization of the McyG A–PCP didomain (78 kDa) using ATP–PPi exchange assays and mass spectrometry revealed that assorted phenylpropanoids are preferentially activated and loaded onto the PCP carrier domain rather than phenylacetate itself. For the first time, thioesters formed in vivo were detected directly using large molecule mass spectrometry. Additionally substrates were cleaved using a type II thioesterase for structural elucidation by small molecule mass spectrometry. Unprecedented features of the McyG A–PCP didomain include the in vivo acylation of the holo PCP with exogenous and endogenous substrates, along with the ability of the apo protein to retain the acyl-AMP intermediate during affinity purification. These results imply that phenylpropanoids are preferentially loaded onto the McyG PCP; however one carbon must be excised following extension of the starter unit with malonyl-CoA in order to generate the expected polyketide chain which leads us to ponder the novel biochemistry by which this occurs.