Abstract
Paralytic shellfish toxins (PSTs) are a group of potent neurotoxic alkaloids that are produced mainly by marine dinoflagellates. PST biosynthesis in dinoflagellates is a discontinuous process that is coupled to the cell cycle. However, little is known about the molecular mechanism underlying this association. Here, we compared global protein expression profiles of a toxigenic dinoflagellate, Alexandrium catenella, collected at four different stages of toxin biosynthesis during the cell cycle, using an isobaric tags for relative and absolute quantification (iTRAQ)-based quantitative proteomic approach. The results showed that toxin biosynthesis occurred mainly in the G1 phase, especially the late G1 phase. In total, 7232 proteins were confidently identified, and 210 proteins exhibited differential expression among the four stages. Proteins involved in protein translation and photosynthetic pigment biosynthesis were significantly upregulated during toxin biosynthesis, indicating close associations among the three processes. Nine toxin-related proteins were detected, and two core toxin biosynthesis proteins, namely, sxtA and sxtI, were identified for the first time in dinoflagellates. Among these proteins, sxtI and ompR were significantly downregulated when toxin biosynthesis stopped, indicating that they played important roles in the regulation of PST biosynthesis. Our study provides new insights into toxin biosynthesis in marine dinoflagellates: nitrogen balance among different biological processes regulates toxin biosynthesis, and that glutamate might play a key modulatory role.
Highlights
Paralytic shellfish toxins (PSTs) are a group of neurotoxic alkaloids that selectively block voltage-gated sodium channels in excitable cells, resulting in approximately 2000 incidents of paralytic shellfish poisoning (PSP) annually worldwide [1]
A group of core genes that are directly involved in toxin biosynthesis, tailor genes that participate in toxin transformation and some additional genes that are responsible for toxin transportation and regulation are characterized, and several toxin-related proteins are identified in cyanobacteria [3,7]
A total of 210 proteins exhibited differential expression in T5 vs. T9, T9 vs. T10 and T10 vs. T11. These proteins are involved in various biological processes, such as PST biosynthesis, protein translation and photosynthetic pigment biosynthesis, and some of these proteins exhibited different variations in these three pairwise comparisons, which might be associated with toxin production
Summary
Paralytic shellfish toxins (PSTs) are a group of neurotoxic alkaloids that selectively block voltage-gated sodium channels in excitable cells, resulting in approximately 2000 incidents of paralytic shellfish poisoning (PSP) annually worldwide [1]. The PST biosynthetic pathway has been unveiled in several species of cyanobacteria, which are another important PST-producing group in addition to dinoflagellates [3,4,5,6]. A group of core genes (sxtA, sxtG, sxtB, sxtD, sxtS, sxtU, sxtH/T and sxtI) that are directly involved in toxin biosynthesis, tailor genes (sxtL, sxtN, sxtX) that participate in toxin transformation and some additional genes that are responsible for toxin transportation and regulation are characterized, and several toxin-related proteins are identified in cyanobacteria [3,7]. Some putative homologs of cyanobacterial toxin genes and proteins have been identified in dinoflagellates [9,10]; only sxtA and sxtG, which participate in the first two biosynthetic steps, have been well characterized [11,12]. The sxtA gene of dinoflagellates has two isoforms: the long isoform contains all the sxtA1–A4 catalytic domains, while the short isoform contains only domains sxtA1–A3, not sxtA4, which is essential for PST biosynthesis [11,13,14,15]
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