The PP2A phosphatase associates with the Arabidopsis TRAPPII tethering complex and dephosphorylates a TRAPPII-derived phosphopeptide in vitro

The transport protein particle II (TRAPPII) complex is a crucial regulator of post-Golgi membrane trafficking in Arabidopsis. Previous research has shown that phosphorylation of the TRAPPII-specific subunit TRS120 by SHAGGY-like kinases controls adaptive growth responses. However, the phosphatases responsible for reversing this phosphorylation have yet to be identified.

Through proteomic analyses, researchers found subunits of Protein Phosphatase 2A (PP2A) within the TRAPPII interactome. These PP2A subunits physically associate with TRAPPII, and genetic studies revealed a link between the two proteins. When TRAPPII function is lost, the relative membrane association of PP2A scaffolding subunits also decreases.

To further investigate this relationship, an in vitro assay was developed to study Arabidopsis PP2A holoenzyme activity using transiently co-expressed and affinity-purified complexes from Nicotiana benthamiana. Structural modeling and interface analysis suggested that a phosphorylated peptide, containing a TRS120 phosphosite cluster, could bind to the PP2A catalytic interface.

Biochemical assays confirmed that a PP2A holoenzyme with the B2 regulatory subunit dephosphorylates this peptide. These findings suggest that a B2-containing PP2A holoenzyme may be a key phosphatase involved in TRS120 regulation, supporting a model where antagonistic SHAGGY-like kinase and PP2A activities connect signaling to membrane trafficking during plant development and environmental adaptation.