By functional coexpression screening of a rat cDNA library in Xenopus oocytes, we have cloned a protein (KCRF: K Channel Regulatory Factor) that reduces currents of several K+ channels: G protein-activated GIRK1/4 (K(ir)3.1/K(ir)3.4), inward rectifier IRK1 (K(ir)2.1), and voltage-dependent K(v)1.1/K(v)beta 1.1. KCRF did not modulate two other K+ channels: ROMK1 (K(ir)1.1) and GIRK1/2 (K(ir)3.1/K(ir)3.2) and the voltage-dependent L-type Ca2+ channels. Western blot analysis showed that KCRF is ubiquitous in rat tissues. Biochemical and electrophysiological experiments revealed that coexpression of KCRF causes a decrease in the level of expression of IRK1 and K(v)1.1/K(v)beta 1.1 proteins in the oocytes.