ERK-2 MAP kinase activation induces inhibitory effects on nuclear protein import in vascular smooth muscle cells. The mechanism and characteristics of this effect of ERK-2 were investigated. An Unusual dose-dependent effect of ERK-2 on nuclear protein import was identified. At higher concentrations (1 mu g/mL) of ERK-2, nuclear protein import was stimulated. whereas lower concentrations (0.04 mu g/mL) inhibited import. Intermediate concentrations exerted intermediate effects. The stimulatory and inhibitory, effects at the 2 different ERK-2 concentrations were observed in both conventional. permeabilized cell assays of nuclear protein import and With in situ microinjection of smooth muscle cells. The biphasic effects of ERK-2 oil import were also found for the other 2 members of the MAPK family. p38 and JNK. Ran-GAP was identified by structural analysis as a candidate target protein responsible for mediating the effects of ERK-2. After pretreatment with high concentrations of ERK-2. RanGAP activity was significantly increased by similar to 50%. In contrast. low concentrations of ERK-2 significantly attenuated RanGAP activity. These results demonstrate that all 3 members of the MAPK family can alter nuclear protein import in opposite directions depending upon the concentration of ERK-2 Used. RanGAP represents the MAP kinase target whereby nuclear transport can be stimulated or inhibited.