Calculation of symmetric multimer structures from NMR data using a priori knowledge of the monomer structure, co-monomer restraints, and interface mapping: The case of leucine zippers.

NMR studies of symmetric multimers are problematic due to the difficulty in distinguishing between intra-, inter-, and co-monomer (mixed) NOE signals. Previously, one of us described a general calculation strategy called dynamic assignment by which this difficulty can be overcome [Nilges, M. (1993) Proteins, 17, 297-309]. Here we describe extensions to the method for handling many co-monomer NOEs and for taking advantege of prior knowledge of monomer structures. The new protocol was developed for the particularly difficult case of leucine zipper (LZ) homodimers, for which the previous protocol proved inefficient. In addition to the problem of dimer symmetry, LZs have a particularly high proportion of co-monomer NOE signals and a high degree of repetition in sequence and structure, leading to significant spectral overlap. Furthermore, the leucine zipper is a rather extended (as opposed to globular) protein domain; accurately determining such a structure based only on the very short distances obtainable by NMR is clearly a challenge to the NMR structure determination method. We have previously shown that, for LZ homodimers, many of the backbone-backbone NOESY cross peaks can be unambiguously assigned as intra-monomer, enabling approximate monomer structures to be calculated. Using model and experimental data sets, we verified that the new protocol converges to the correct dimer structure. The results show that short-range NMR distance data can be sufficient to define accurately the extended LZ. The protocol has been used to derive a novel solution structure of the c-Jun LZ domain. Based on these calculations, we propose the protocol as a prototype for the general case of symmetric multimers where the monomer structure is known.