1. Institute of Enzymology, Biological Research Center, Hungarian Academy of Sciences, Budapest, H-1113, Hungary, Department of Biology, Faculty of Medicine, University of Szeged, Szeged, H-6720, Hungary, Department of Pharmaceutical Chemistry, School of Pharmacy, University of CaliforniaSan Francisco, San Francisco, California 94143-0446, and Department of Anatomy, Cell and Developmental Biology, Faculty of Science, Eötvös Loránd University, Budapest, H-1117, Hungary