Mannitol metabolism in brown algae involves a new phosphatase family

Area of Science:

• Marine biology
• Biochemistry
• Molecular biology

Background:

• Brown algae inhabit harsh intertidal zones, necessitating unique metabolic adaptations.
• The mannitol cycle is crucial for brown algae, serving roles in carbon storage, osmoregulation, and antioxidant defense.
• Mannitol is synthesized from fructose-6-phosphate via mannitol-1-phosphate dehydrogenase and mannitol-1-phosphatase (M1Pase).

Purpose of the Study:

• To identify and characterize genes encoding M1Pase activity in the brown alga Ectocarpus siliculosus.
• To investigate the enzymatic function and substrate specificity of potential M1Pase candidates.
• To analyze the expression patterns and evolutionary significance of these M1Pase genes.

Main Methods:

• Genome analysis of Ectocarpus siliculosus to identify candidate M1Pase genes (EsM1Pase1 and EsM1Pase2).
• Heterologous expression of candidate genes in Escherichia coli for biochemical characterization.
• Enzyme activity assays using mannitol-1-phosphate and other hexose monophosphates.
• Gene expression analysis under diurnal cycles.
• Sequence analysis and 3D homology modeling.

Main Results:

• Two haloacid dehalogenase (HAD)-like genes, EsM1Pase1 and EsM1Pase2, were identified as potential M1Pase candidates.
• Recombinant EsM1Pase2 demonstrated specific activity in hydrolyzing mannitol-1-phosphate to mannitol, without acting on other hexose monophosphates.
• Transcription of both EsM1Pase genes was found to be influenced by the diurnal cycle.
• EsM1Pases and related Prasinophyte orthologues represent a novel phosphatase family within the HAD superfamily, characterized by unique substrate specificity.

Conclusions:

• EsM1Pase2 is confirmed to encode functional mannitol-1-phosphatase activity in Ectocarpus siliculosus.
• This study provides the first characterization of a gene encoding M1Pase in photosynthetic organisms.
• The findings reveal a new family of phosphatases with distinct substrate specificity, expanding our understanding of the HAD superfamily and brown algal metabolism.