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RAN: AN ATYPICAL GTPASE Mark G. Rush and Peter D'Eustachio New York University School o/Medicine. Department,o/Biochemistry New York NY 10016 ABSTRACT GTPases, proteins that bind and hydrolyze GTP (guanosine triphos- phate) are critical regulators of many metabolic pathways. Although these proteins are enzymes that catalyze the hydrolysis of GTP to GDP + Pi, their primary function is not the hydrolysis of GTP per se, but rather the coupling of this hydrolysis to metabolic regulation. Such coupling is gen- erally achieved through the interaction of the GTP-bound form of the GTPase with proteins known as *effectors. Effectors are often enzymes whose activities are modulated by the GTPase. However, effectors can also be structural proteins involved in assembling intracellular macromo- lecular complexes, such as actin filaments and microtubules, as well as proteins involved in the intracellular transport of proteins and RNAs. In- deed, the subject of this anthology, the small GTPase Ran, may exert most or all of its regulatory functions by interacting with non-enzyme effectors. This property of Ran distinguishes it from other well studied GTPases, and has resulted in the elucidation of novel mechanisms of Ran action that are quite distinct from previously established paradigms of GTPase function. 1. INTRODUCTION The Ras-related nuclear protein Ran is a highly conserved (80% identity among yeasts and humans) member of the Ras superfamily of small GTP binding and hydrolyzing proteins.