Phosphoregulation of Mitochondrial Survivin in Cancer Cells

The Wheatley lab has been working for over a decade on the cancer-associated protein survivin. This fascinating little protein is essential for mitosis and cell division, but also prevents cells from dying (1,2). It is overexpressed in all cancers and its abundance correlates with poor patient prognosis, resistance to conventional chemotherapy and radiation, and metastasis.

Phosphotidylethanolamine (PE) is a phospholipid that is critical to membrane architecture: it facilitates positive curvature by virtue of its small head group. Consequently PE exposure at the plasma membrane (PM) of the cleavage furrow is critical for completion of cell division, and its high expression in mitochondrial cristae governs mitochondrial integrity, ultimately impacting on the metabolic needs of the cell, and cellular response to apoptotic stimuli (3). PE is also a critical regulator of autophagy, the recycling pathway initiated by cells during stressful conditions (4). At its outset autophagy is a pro-survival mechanism, but if it continues unabated, it ultimately causes cell death. In animal cells 80% of the de novo pool of PE is synthesised in the mitochondria by decarboxylating phosphatidylserine (PS), which is imported from the endoplasmic reticulum. Conversion of PS to PE is mediated by the inner mitochondrial membrane resident, phosphatidylserine decarboxylase (PSD), and if PSD is depleted, the PS/PE ratio rises, mitochondrial morphology becomes perturbed, with growth rate, metabolism and apoptosis affected (3).

Recently we discovered that survivin interacts directly with PSD in the mitochondria of cancer cells (5). Our initial data suggest that it is a negative regulator of PE production, and we hypothesise that survivin alters membrane architecture by limiting PE availability. In this manner survivin expression can influence many fundamental processes, including cell divisio, mitochondrial health, metabolic reprogamming, and may be able to prevent cells from dying through excessive autophagy. This novel molecular insight suggests that the apparently mutliple and disparate roles of survivin, may be fundamentally linked at this protein-lipid interface. This hypothesis opens up a completely new perspective on survivin function and how it contributes to cancer and other metabolic disorders.

In this project, the regulation of survivin-PSD interaction by phosphorylation (cdk1/p38) will be investigated. In preliminary work we have mapped the interaction between survivin and PSD to the baculovirus inhibitor of apoptosis repeat (BIR) domain of survivin and the decarboxylase domain of PSD (5). Within the BIR domain there is a unique cdk1 site (T34), and previous work from our lab has shown that cells expressing T34 mutants, grow slowly, are highly resistant to apoptosis and are insentive to irradiation (6). Moreover, by electron microscopy we have discovered that mitochondrial integrity is compromised in these cells. Thus the goal of this project is to investigate the impact cdk1/p38 phosphorylation of T34 on survivin-PSD interaction, PE production and to document the downstream consequences in terms of cell division, metabolism, response to apoptotic stimuli including radiation, and excessive autophagy.