DYNAMICAL BEHAVIOR OF THE HIV-1 NUCLEOCAPSID PROTEIN

The HIV-1 nucleocapsid protein (NC) contains two CCHC-type zinc knuckl e domains that are essential for genome recognition, packaging and inf ectivity. The solution structure of the protein has been determined in dependently by three groups. Although the structures of the individual zinc knuckle domains are similar, two of the studies indicated that t he knuckles behave as independently folded, non-interacting domains co nnected by a flexible tether, whereas one study revealed the presence of interknuckle NOE cross-peaks, which were interpreted in terms of a more compact structure in which the knuckles are in close proximity. W e have collected multidimensional NMR data for the recombinant, isotop ically labeled HIV-1 NC protein, and confirmed the presence of weak in terknuckle NOEs. However, the NOE data are not consistent with a singl e protein conformation. N-15 NMR relaxation studies reveal that the tw o zinc knuckle domains possess different effective rotational correlat ion times, indicating that the knuckles are not tumbling as a single g lobular domain. Ln addition, the H-1 NMR chemical shifts of isolated z inc knuckle peptides are very similar to those of the intact protein. The combined results indicate that the interknuckle interactions, whic h involve the close;approach of the side-chains of Phe16 and Trp37, ar e transitory. The solution behavior of NC may be best considered as a rapid equilibrium between conformations with weakly interacting and no n-interacting knuckle domains. This inherent conformational flexibilit y may be functionally important, enabling adaptive binding of NC to di fferent recognition elements within the HIV-1 Psi-RNA packaging signal . (C) 1998 Academic Press Limited.