# PP2A/B55alpha substrate specificity and function in cell cycle progression

> **NIH NIH R01** · TEMPLE UNIV OF THE COMMONWEALTH · 2024 · $661,956

## Abstract

The goal of this proposal is to fully delineate the molecular mechanisms by which the prominent serine/threonine
Protein Phosphatase 2 (PP2A) holoenzyme, specifically PP2A/B55α, recognizes its substrates through a new
helical motif and to use this information to identify direct PP2A/B55α substrates with functions in mitogenic
signaling and early cell cycle progression. Protein phosphorylation is an essential regulatory mechanism in cell
signaling and the regulation of the cell cycle and it is altered in cancer, Alzheimer, and other diseases. A large
fraction of dephosphorylation is mediated by distinct PP2A trimeric holoenzymes. These consist of a scaffold
protein that connects the catalytic subunit to the variable regulatory subunit, which mediates substrate specificity.
The most abundant holoenzyme is PP2A/B55α. Its activity has been implicated in mitogenic signaling and early
cell cycle progression, but details remain unclear. Initially, it was believed that PP2A/B55α recruited substrates
through charge-charge interactions between the surface of B55α and substrates. However, our groundbreaking
discoveries challenged this notion. We identified a conserved amino acid motif in various proteins, including the
p107 substrate and the FAM122A inhibitor, that folds as an a-helix. This helical motif is essential for p107
dephosphorylation and for FAM122A binding via similar interactions with residues in a groove on the top of B55α.
This mechanism is distinct from that used by other Ser/Thr phosphatases that bind short linear motifs in
intrinsically disordered protein regions. Gap: Despite this advance, we remain unable to identify other
PP2A/B55α substrates using this short helical sequence. This is due to the lack of a comprehensive assessment
of amino acid variability within the helical motif sequence and incomplete detail of the motif’s structural plasticity.
As a result, it remains extremely difficult to identify new substrates, to distinguish correctly vs incorrectly assigned
substrates, and to demonstrate the veracity of correctly assigned substrates. We hypothesize that a helical
region with a defined length and a variable conserved amino acid sequence in PP2A/B55α substrates mediates
binding to the top groove of B55α via specific electrostatic and hydrophobic interactions. This is a primary
mechanism for substrate recognition that modulates signaling through the cell cycle. These will be tested with
two aims: (1) To define PP2A/B55α substrate specificity via a short helical motif with defined sequences. (2) To
dissect the PP2A/B55α substrate interplay in mitogenic signaling linked to the cell cycle Restriction Point (RP).
DEIA Aims: Diversity among Temple University undergraduates is excellent, but the transition of
underrepresented minority (URM) students to biomedical science research careers or graduate schools is still
severely limited. Our goal is to implement a program we have been piloting that is geared toward eliminating
barriers to jo...

## Key facts

- **NIH application ID:** 10946400
- **Project number:** 1R01GM155497-01
- **Recipient organization:** TEMPLE UNIV OF THE COMMONWEALTH
- **Principal Investigator:** Xavier Grana
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2024
- **Award amount:** $661,956
- **Award type:** 1
- **Project period:** 2024-08-01 → 2025-06-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10946400

## Citation

> US National Institutes of Health, RePORTER application 10946400, PP2A/B55alpha substrate specificity and function in cell cycle progression (1R01GM155497-01). Retrieved via AI Analytics 2026-06-25 from https://api.ai-analytics.org/grant/nih/10946400. Licensed CC0.

---

*[NIH grants dataset](/datasets/nih-grants) · CC0 1.0*