Control mechanisms in germ cells mediated by p53 family proteins.

Germ cells are totipotent and, in principle, immortal as they are the source for new germ cells in each generation. This very special role requires tight quality control systems. The p53 protein family constitutes one of the most important quality surveillance systems in cells. Whereas p53 has become famous for its role as the guardian of the genome in its function as the most important somatic tumor suppressor, p63 has been nicknamed 'guardian of the female germ line'. p63 is strongly expressed in resting oocytes and responsible for eliminating those that carry DNA double-strand breaks. The third family member, p73, acts later during oocyte and embryo development by ensuring correct assembly of the spindle assembly checkpoint. In addition to its role in the female germ line, p73 regulates cell-cell contacts between developing sperm cells and supporting somatic cells in the male germ line. Here, we review the involvement of the p53 protein family in the development of germ cells with a focus on quality control in the female germ line and discuss medical implications for cancer patients.

Molecular crowding drives active Pin1 into nonspecific complexes with endogenous proteins prior to substrate recognition.

Proteins and nucleic acids maintain the crowded interior of a living cell and can reach concentrations in the order of 200-400 g/L which affects the physicochemical parameters of the environment, such as viscosity and hydrodynamic as well as nonspecific strong repulsive and weak attractive interactions. Dynamics, structure, and activity of macromolecules were demonstrated to be affected by these parameters. However, it remains controversially debated, which of these factors are the dominant cause for the observed alterations in vivo. In this study we investigated the globular folded peptidyl-prolyl isomerase Pin1 in Xenopus laevis oocytes and in native-like crowded oocyte extract by in-cell NMR spectroscopy. We show that active Pin1 is driven into nonspecific weak attractive interactions with intracellular proteins prior to substrate recognition. The substrate recognition site of Pin1 performs specific and nonspecific attractive interactions. Phosphorylation of the WW domain at Ser16 by PKA abrogates both substrate recognition and the nonspecific interactions with the endogenous proteins. Our results validate the hypothesis formulated by McConkey that the majority of globular folded proteins with surface charge properties close to neutral under physiological conditions reside in macromolecular complexes with other sticky proteins due to molecular crowding. In addition, we demonstrate that commonly used synthetic crowding agents like Ficoll 70 are not suitable to mimic the intracellular environment due to their incapability to simulate biologically important weak attractive interactions.

Regulation of the Activity in the p53 Family Depends on the Organization of the Transactivation Domain.

Despite high sequence homology among the p53 family members, the regulation of their transactivation potential is based on strikingly different mechanisms. Previous studies revealed that the activity of TAp63α is regulated via an autoinhibitory mechanism that keeps inactive TAp63α in a dimeric conformation. While all p73 isoforms are constitutive tetramers, their basal activity is much lower compared with tetrameric TAp63. We show that the dimeric state of TAp63α not only reduces DNA binding affinity, but also suppresses interaction with the acetyltransferase p300. Exchange of the transactivation domains is sufficient to transfer the regulatory characteristics between p63 and p73. Structure determination of the transactivation domains of p63 and p73 in complex with the p300 Taz2 domain further revealed that, in contrast to p53 and p73, p63 has a single transactivation domain. Sequences essential for stabilizing the closed dimer of TAp63α have evolved into a second transactivation domain in p73 and p53.