Regulation of protein phosphatase 2A by proteasomal degradation

Protein phosphatase 2A (PP2A), a ubiquitous and pleiotropic regulator of intracellular signaling, is composed of a core dimer (A scaffolding and C catalytic subunits) bound to a variable (B) regulatory subunit of either the B, B' or B" families. Further genetic expansion and alternative splicing within each B subunit family affords the enzyme tremendous functional heterogeneity as PP2A contributes dozens of heterotrimers with varying subcellular locations and cellular substrates dictated by the variable B subunit. B'β is a brain-specific PP2A regulatory subunit that mediates dephosphorylation of Ca2+/calmodulin-dependent protein kinase II and tyrosine hydroxylase. Unbiased proteomic screens for B'β interactors identified Cullin3 (Cul3), a scaffolding component of E3 ubiquitin ligase complexes, and the previously uncharacterized Kelch-like 15 (KLHL15). KLHL15 is one of more than 40 Kelch-like proteins, many of which have been defined as adaptors for the recruitment of substrates to Cul3-based E3 ubiquitin ligases. KLHL15/Cul3 specifically targets B'β to promote protein turnover via ubiquitination and proteasomal degradation. Comparison of KLHL15 and B'β expression profiles suggest that the E3 ligase adaptor contributes to selective expression of the PP2A/B'β holoenzyme in the brain. Mapping of KLHL15 residues critical for Cul3 binding and protein dimerization indicate two distinct and independent functions of KLHL15's N-terminal BTB domain while similar analysis of the C-terminal kelch domain identifies a B'β-specific binding core. While B' regulatory subunit association with the AC dimer is mediated by a highly conserved inner core of roughly 400 amino acids, the divergent N-terminus of B'β is found to be both necessary and sufficient for KLHL15-mediated degradation, with Tyr52 having an obligatory role, underlying the selective association of KLHL15 with the B'β regulatory subunit only. KLHL15 can interact with not only the monomeric version of B'β but also the more stable, PP2A/B'β heterotrimer. However, proteasomal targeting is reserved for the B subunit only. The loss of B'β promotes an exchange of B subunits and a reciprocal upregulation of alternative B subunit-containing heterotrimers. That is, excess KLHL15 may not only downregulate B'β-guided PP2A dephosphorylation activity, but moreover, may upregulate PP2A activity dictated by alternative B subunits. Taken together, these data suggest regulatory subunit-specific ubiquitination and proteasomal degradation as a novel mechanism for controlling total cellular PP2A activity.