Protein Involved in Initiation 1 Interaction With Starch Synthase 4 From <i>Arabidopsis thaliana</i> Induces Inhibition of Elongating 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 SS4's enzymatic activity. Our findings reveal that PII1 contains a long coiled-coil domain that specifically interacts with SS4, leading to modification of SS4's glucan elongation activity. Importantly, this interaction is specific to SS4 and does not affect other known synthases, suggesting a targeted regulatory mechanism probably through a dimerization domain. This work describes the structural specificities of PII1 and SS4 and reveals a possible function for PII1 in the initiation complex.