NUCLEAR-LOCALIZATION SIGNAL BINDING-PROTEINS IN HIGHER-PLANT NUCLEI

The import of proteins into the nucleus is a vital process that is med iated by proteins which specifically recognize nuclear localization si gnals (NLSs). These factors have not been identified in plants. Previo usly, we demonstrated that higher plants possess a low-affinity bindin g site at the nuclear pore that specifically binds to several classes of functional NLSs. By the use of crosslinking reagents and a radiolab eled peptide to the bipartite NLS from the endogenous plant transcript ion factor Opaque2, two NLS binding proteins (NBPs) of 50-60 kDa and a t least two NBPs of 30-40 kDa were identified. Competition studies ind icated that labeling was specific for the functional NLS but not a mut ant NLS impaired in vivo or a peptide unrelated to NLSs. Also, the app arent dissociation constant (100-300 mu M) for labeling was similar to that of the binding site. Proteins of similar mass mere labeled with two different crosslinking reagents, and concentration and time studie s indicated that these NBPs were distinct proteins and not aggregates. Treatment with salt, detergent, or urea before or during NLS binding demonstrated that the properties of the binding site and the NBPs were identical. This tight correlation strongly indicates that some or all of the NBPs constitute the nuclear pore binding site. Overall, our re sults indicate that some components of NLS recognition are located at the nuclear pores in higher plants.