PROJECT SUMMARY SUMOs (small ubiquitin-related modifiers) are proteins that are posttranslationally conjugated to other proteins, thereby regulating a wide range of essential functions critical for normal cell growth, differentiation and survival. Notably, sumoylation has been directly linked to the development and progression of a variety of human diseases, including neurodegeneration, stroke, heart disease, cleft lip/palate and cancer. During the past ten years enormous progress has been made in elucidating the molecular mechanisms involved in conjugating SUMOs to target proteins and in identifying the cellular processes regulated by sumoylation. Despite these insights, however, fundamental questions about how sumoylation actually affects modified proteins at the molecular level, and how these effects are translated into control of cell function, remain to be addressed. In many cases, the effects of sumoylation on target proteins remain unknown. In addition, how sumoylation is able to elicit unique fates upon conjugation to different proteins is also poorly understood. We will use the budding yeast, S. cerevisiae, as a model organism for studying the essential functions of sumoylation. A goal of the proposed studies is develop a high-resolution structure/function map of SUMO. This goal will be achieved through the synthesis of a comprehensive library of yeast SUMO mutants and characterization of these mutants using a variety of assays designed to probe SUMO structure/function relationships both biochemically and genetically. With this library, we will test the hypothesis that SUMO represents a versatile and variable signal with distinct surface residues affecting interactions with distinct regulatory factors and downstream SUMO-binding proteins. We will also test the hypothesis that sumoylation functions in a chaperone-like capacity to protect proteins from damage and aggregation caused by the effects of cytotoxic cell stresses.