Selective nuclear import is mediated by nuclear localization signals (NLSs) and cognate transport factors known as karyopherins or importins. Karyopherin alpha recognizes "classical" monopartite and bipartite NLSs. We report the crystal structure of a 50 kDa fragment of the 60 kDa yeast karyopherin alpha, in the absence and presence of a monopartite NLS peptide at 2.2 A and 2.8 A resolution, respectively. The structure shows a tandem array of ten armadillo repeats, organized in a right-handed superhelix of helices. Binding of the NLS peptide occurs at two sites within a helical surface groove that is lined by conserved residues. The structure reveals the determinants of NLS specificity and suggests a model for the recognition of bipartite NLSs.

Proteins containing Dbl homology (DH) domains activate Rho-family GTPases by functioning as specific guanine nucleotide exchange factors. All known DH domains have associated C-terminal pleckstrin homology (PH) domains that are implicated in targeting and regulatory functions. The crystal structure of a fragment of the human Son of sevenless protein containing the DH and PH domains has been determined at 2.3 A resolution. The entirely alpha-helical DH domain is unrelated in architecture to other nucleotide exchange factors. The active site of the DH domain, identified on the basis of sequence conservation and structural features, lies near the interface between the DH and PH domains. The structure suggests that ligation of the PH domain will be coupled structurally to the GTPase binding site.

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