Mapping protein-protein interactions is an invaluable tool for understanding protein function. Here, we report the first large-scale study of protein-protein interactions in human cells using a mass spectrometry-based approach. The study maps protein interactions for 338 bait proteins that were selected based on known or suspected disease and functional associations. Large-scale immunoprecipitation of Flag-tagged versions of these proteins followed by LC-ESI-MS/MS analysis resulted in the identification of 24 540 potential protein interactions. False positives and redundant hits were filtered out using empirical criteria and a calculated interaction confidence score, producing a data set of 6463 interactions between 2235 distinct proteins. This data set was further cross-validated using previously published and predicted human protein interactions. In-depth mining of the data set shows that it represents a valuable source of novel protein-protein interactions with relevance to human diseases. In addition, via our preliminary analysis, we report many novel protein interactions and pathway associations.

Xenopus laevis oocytes are physiologically arrested at G2 of meiosis I. Resumption of meiosis, or oocyte maturation, is triggered by progesterone. Progesterone-induced Xenopus oocyte maturation is mediated via an extranuclear receptor and is independent of gene transcription. The identity of this extranuclear oocyte progesterone receptor (PR), however, has remained a longstanding problem. We have isolated the amphibian homologue of human PR from a Xenopus oocyte cDNA library. The cloned Xenopus progesterone receptor (xPR) functioned in heterologous cells as a progesterone-regulated transcription activator. However, endogenous xPR was excluded from the oocyte nucleus and instead appeared to be a cytosolic protein not associated with any membrane structures. Injection of xPR mRNA into Xenopus oocytes accelerated the progesterone-induced oocyte maturation and reduced the required concentrations of progesterone. In enucleated oocytes, xPR accelerated the progesterone-induced mitogenactivated protein kinase activation. These data suggest that xPR is the long sought after Xenopus oocyte receptor responsible for progesterone-induced oocyte maturation.

An endogenous G protein betagamma dimer, likely including Xenopus Gbeta1, is responsible for maintaining oocyte meiosis arrest. Resumption of meiosis is induced by Gbetagamma scavengers in vitro or, naturally, by progesterone via a mechanism that suppresses the release of Gbetagamma.