UV-crosslinking assay UV-crosslinking assay was performed as described previously (Huang and Lai, 1999) . In brief, DBT cell extracts (30 mg protein), 200 mg/ml tRNA and 10 4 c.p.m. of an in vitro transcribed, 32 P-labeled negativestrand MHV 5¢-end RNA (182 bp) were incubated for 10 min at 30°C. Increasing amounts of puri®ed GST (0, 0.5, 1.5 and 5 ng) or recombinant GST±hnRNP A1 fusion protein (0, 1, 3 and 10 ng) were included in the reaction to compete with the endogenous hnRNP A1 for binding. The reaction mixture was placed on ice and UV-irradiated in a UV Stratalinker 2400 (Stratagene) for 10 min, followed by digestion with 400 mg/ml RNase A for 15 min at 37°C. The protein±RNA complexes were then separated on a 10% SDS±polyacrylamide gel and visualized by autoradiography.