1. Centre de Génétique Moléculaire, CNRS, 91198 Gif-sur-Yvette Cedex, 2Université Paris-Sud, 91405 Orsay, France, 3Mass Spectrometry Laboratory, Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Warsaw and 4Institute of Genetics and Biotechnology, Department of Biology, Warsaw University, Warsaw, Poland