Phosphoglucan phosphatase function sheds light on starch degradation

• Phosphoglucan phosphatases are novel enzymes that dephosphorylate carbohydrates in eukaryotes.
• Reversible glucan phosphorylation initiates the remobilization of leaf starch at night.
• SEX4 dephosphorylates both C3- and C6-bound phosphate on amylopectin, whereas LSF2 is C3 specific.
• LSF1 may recruit starch degradative enzymes to the granule surface.

Phosphoglucan phosphatases are novel enzymes that remove phosphates from complex carbohydrates. In plants, these proteins are vital components in the remobilization of leaf starch at night. Breakdown of starch is initiated through reversible glucan phosphorylation to disrupt the semi-crystalline starch structure at the granule surface. The phosphoglucan phosphatases starch excess 4 (SEX4) and like-SEX4 2 (LSF2) dephosphorylate glucans to provide access for amylases that release maltose and glucose from starch. Another phosphatase, LSF1, is a putative inactive scaffold protein that may act as regulator of starch degradative enzymes at the granule surface. Absence of these phosphatases disrupts starch breakdown, resulting in plants accumulating excess starch. Here, we describe recent advances in understanding the biochemical and structural properties of each of these starch phosphatases.