PII1/MRC interaction with Starch Synthase 4 (SS4) from Arabidopsis thaliana inhibits SS4 enzymatic activity

Starch is the major energy storage compound in plants. It accumulates in the form of insoluble, partly crystalline granules whose number and shape are specific to each plant species. These characteristics are defined very early in starch biosynthesis, at the initiation stage. Starch biosynthesis initiation is a complex process that relies on the coordinated action of several proteins that interact together in the so-called complex of initiation. Starch Synthase 4 (SS4) is the only initiation protein with enzymatic activity. It catalyzes the formation of glucan primers, which serve as substrates for the enzymatic machinery that synthesizes starch granules. Previous studies have highlighted the importance of interactions between SS4 and regulatory proteins in this process. Among them, Protein Involved in Initiation 1 (PII1) interacts with SS4 but its function is not yet established. In this study, we explored the structural and functional implications of PII1 on SS4s enzymatic activity. Our findings reveal that PII1 contains a long coiled-coil domain that specifically interacts with SS4, leading to significant inhibition of SS4s glucan elongation activity. Importantly, this inhibition is specific to SS4 and does not affect other known synthases, suggesting a targeted regulatory mechanism. This work describes the structural specificities of PII1 and SS4 and reveals a function for PII1 in the initiation complex. These results allow us to re-examine these complex mechanisms and propose new hypotheses about the important steps in the initiation of starch biosynthesis.